A Magical Roommate, by the same author as To Prevent World Peace, is by a personal friend, and is about a girl from a fantasy world who is obliged to attend college on Earth.
Bardsworth is about a boy from Earth who is invited to attend college in a fantasy world.
Basic Instructions will tell you how to live your life.
The Order of the Stick is about a D&D adventuring party who are aware of the game mechanics that govern their lives.
Dinosaur Comics is about some dinosaurs and has remarkably consistent artwork.
Dreamless is a beautifully illustrated comic about two people who see through each other's eyes when asleep. It has ended.
Erfworld is about an Earth native whose roleplaying game skills make him a warlord in a suspiciously gamelike world.
Flaky Pastry is about a goblin, a cat-eared individual, and an elf who are roommates, and who witness and participate in an assortment of wacky happenings.
Goblins is a gaming comic told from the perspective of the standard-issue villains in most D&D campaigns.
Gunnerkrigg Court is of uncertain genre, and concerns the explorations and revelations of one Antimony Carver.
Khaos Komix is a multi-threaded romance consisting almost entirely of alternative-sexuality characters.
Lackadaisy Cats is a marvelously drawn comic set during Prohibition, and also, everyone is a cat.
Ozy and Millie, by the same author as Raine Dog, is a cute comic in a furry world about a Zen arctic fox boy and spastic nonarctic fox girl who are best friends.
Questionable Content contains comparatively little questionable content, and is a slice of life about some friends and their tiny robots.
Saturday Morning Breakfast Cereal updates frequently; the humor is lowbrow, the topics run the gamut.
Skin Deep is a richly illustrated group of stories about mythical creatures who hide in plain sight.
Skin Horse is about a team of government agents including a talking dog, a zombie, and a transvestite.
Something Positive is a slice of life with a unique ratio of snark to gelatinous cat. Its sibling comics (available from the same site) are also recommended.
To Prevent World Peace, by the same author as A Magical Roommate, is about villains trying to protect the world from overpowered and overzealous magical girls.
Subnormality defies description, but it has the Sphinx in it.
Yet Another Fantasy Gamer Comic is about the doings of dungeon denizens, from goblins and orcs to beholders and drow.
xkcd is a webcomic of romance, sarcasm, math, and language.
Casey and Andy has ended, and is about mad scientist roommates who periodically die.
Minus no longer updates, and is about a little girl who, interestingly enough, is omnipotent.
Negative One is by a personal friend, and is about a number of characters trying to get along in the world despite variously fantastical interferences.
Get Medieval is about aliens who crash-land on Earth during medieval times. It has ended, but it's running old strips with commentary now.
Narbonic has ended, but was about a mad scientist, as well as her employees, test subjects, rivals, and casualties.
One Over Zero has ended, and was a relentlessly self-referential comic with considerable philosophical depth.