Brian Parker from Abstract Gender can change his sex whenever he likes. His friend Ryan, who was in the same experiment, wasn't quite so lucky — he also became a girl, but can't change back.
Most characters in The Beast Legion transform into beasts. Some examples can be found 12,3.
In Beyond the Canopy, Jojo is a semi-solid being who can alter his shape and proportions at will.
The Mons in But I'm a Cat Person. Unless ordered by their Master to take a specific form, they can shift freely. Available forms include any kind of human and any variation on their particular animal.
Myan in Cat Nine can turn into any animal by using her magic collar.
Drel from Corner Alley 13 is a magically altered werewolf with this ability. The prequel Weakness reveals that he must remain at a set mass while transforming.
The Dragon and Cubi races in Dan and Mab's Furry Adventures are all natural shapeshifters, though learning to use it effectively takes practice. In addition Mad Scientist Jyrras invents "patches" (a pun on the use of software patches in the original Furcadia) which can modify one's appearance.
Darths & Droids takes the idea of shapeshifters that was used briefly in the Star Wars movies and runs with it. This is a comic using screenshots from those movies for its panels, but the existence of shapeshifters is one way it can change things in the story; just because two characters look totally different doesn't mean they're not actually the same shapeshifting character in different roles.
In Doctor Who Regenerated we are informed of a huge group of aliens who can do this, but as of yet, we've only met one of them, whose default form resembles a beetle.
Ariel from Drowtales not only has Prehensile Hair but the ability to completely copy a person's shape and voice. She once used this this power to diguise herself as Faen, get captured and then release everyone imprisoned with her.
Felucca's power in Earthsong. She shapeshifts into a huge dragon.
Lisa from Experimental Comic Kotone, a fox-girl, has this ability. According to a legend among her people, a fox-person's true love should be able to see through their disguises. This has caused her much consternation and confusion, since everyone has seen through her disguises.
Familiar Ground: The horse. At first, only with the help of his paladin, but later he acquires it independently.
In general, transformation magic is extremely common in the comic. Almost every magically-inclined character can shapeshift in some way, even when it's not their main power: Nanase can shift her hairstyle, Rhoda can shapeshift to disguise herself, and that's not even getting into the multitude of minor characters who can shapeshift. In part, this is because magic runs on Personality Powers and most people want to change something about themselves, but also it's clearly Author Appeal.
Grace is an escaped partly alien lab experiment who can assume several human or part-human forms. While she can only take on forms she has had before (or combine them together), she can effectively create new forms whenever she needs to by means of her Mad Scientist boyfriend's Transformation Gun. Even in a cast of shapeshifters, Grace is the most effective and flexible of the main cast.
Thanks to a botched Magical Awakening brought on by a magic artifact (long story), Elliot has the ability to turn into a girl at will. At first he could only look like Ellen, although he later gained the ability look like any girl as long as he concentrates. However, thanks to said improper Awakening, Elliot spent a few months as an Involuntary Shapeshifter since he was building up excess magic energy, which would cause him to spontaneously become a girl if he didn't burn the energy off beforehand.
Ellen has a form of this herself. She can zap someone (animals included) with an invisible beam and turn into a perfect replica of them. At one point, she even uses it to copy Nanase's Guardian Angel form.
Gunnerkrigg Court has a few. Mort is a ghost who can change his form at will. Reynardine can currently alter the shape of the stuffed toy he's possessing. Ysengrin received new arms from Coyote: arms that he can shape into weapons. Coyote's size and proportions are constantly changing.
A few characters in Imaginatives can change their form at will, such as Excalibur, who can change into his lycanthropic anthro wolf form at will, and the sentient plush Squeaky, who can change into any dinosaur or dinosaur-like (such as pterosaurs for flight) as he pleases.
Princess Voluptua in The Inexplicable Adventures of Bob! uses a shapeshifting device to appear human, her natural form being insectoid. It has been stated that the unrealistic curvaceousness of her assumed form was unintentional on her part, but she shows no signs of wanting to reprogram the device to correct the error.
In The Kingfisher some vampires can turn into animals, animalistic monster forms, mist, and/or some kind of shadowy form. These abilities are all at will and seem limited by the vampire's lineage.
Decoy Octopus from The Last Days of FOXHOUND can take on the shape of anyone. However, to do so, he needs to drink some of their blood (with a special ingredient added).
Supposedly an angelic ability in Misfile, only Cassiel has used it so far on screen and she isn't so great at it.
Ghosts get this power in 1/0. Though Mock is the only one who uses it.
The Order of the Stick has as a prime example with Sabine the Succubus, who can take various humanoid forms.
Since the webcomic is based on D&D, you can easily find other examples, such as the druidic and magical versions.
Pebble and Wren: Pebble can shapeshift, but whatever he turns into still has his stripes.
Isla Grace from Professor Amazing and the Incredible Golden Fox: receives a fox-shaped engagement ring from her husband-to-be, Parker, which gifts her with the ability to transform into a fox (both anthropomorphic and full fox versions). She then feels it's only appropriate to put her new powers to use in the service of her community.
Reversed Star: In a mock article, some aliens are stated to be able to do this.
Schlock of Schlock Mercenary is one, although unlike some of the other examples possessing no fixed form, he's not good at bipedal motion.
Scoob and Shag: Courage's Ballyhoo, "On Demand", allows him to transform into different objects, including weapons. Strip 108 reveals that this is not limited to inanimate objects: for example, he can turn into a human.
The Senkari can change their forms to a limited degree. This appears to be limited to their wings and certain wounds so far.
Rachael from Silver Bullet Nights can shift her hands into claws and her boyfriend shifts between a solid form and a spirit one at will.
Kieri from Slightly Damned was cursed into a form of a snow bunny, however, the being responsible was in too much of a hurry to do a particularly good job of it and now she has a limited ability to shift between the two forms at will.
Sluggy Freelance: Demons possessing humans can switch the possessed body's shape between the human form and a creepy half-demon one. When K'Z'K is first introduced possessing Gwynn in "K'Z'K", he's able to switch between Gwynn's normal form and a giant horned monster (which does not look like this true form), and to reform himself after being ground to hamburger meat. Later in "The Immortal King" Bun-bun challenges him in his true giant bug demon form and tries to stay out of his reach by going around to the back, where none of K'Z'K's arms or claws can reach; K'Z'K responds by showing he can shift his form however he likes — pointing out he's more of a concept than a creature — and opts for a demon-bug-bunny form to fight Bun-bun on more equal terms for the fun of it.
In Sonic the Comic – Online! a few characters can do this, first the Nightmare Creature who changes into Johnny and Johnny as a zombie, then the Neo Metallix who can change his form and assume other identities such as Sonic or Tails which he uses to frame Sonic for his crimes, there is also the Gizoid which has the ability to perfectly mimic the appearance and fighting style of its adversaries.
Catan from Tower of God, in his Shapeshifter Default Form a black blob with the silhouette of a bell curve, can transform free and uses that ability to fight. This also becomes his downfall, as Khun sucks him up with his Bag of Holding while he is in between forms.
Unsounded: Lady Ilganyag's default form is a giant crow with red eyes and six pairs of breasts, but she can open a multitude of red eyes in her wings which can shift to look like vulvas if she so chooses, and she can have the top half of a red skinned woman-like form crawl from the bird's beak to become more of a crow centaur looking thing. While in the khert she can also adopt the form of various memories of women, with her red eyes giving her away.
Animal people in Wilde Life can transform between human and animal forms. Cliff, one of the main characters, can turn into a wolf. Raven can also turn into other kinds of humans; he can transform into a boy and even into a girl once, or into a monster.
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