Online Comics

There are thousands out there; these I read often. If I'm leaving out your favorite comic, let me know!
Penny Arcade - Tycho's got wit sharp enough to "gut me from crotch to sternum like a Rainbow Trout" as Max Cannon would say.  Gabe draws the art and plays Bad Cop. 
Player Vs. Player - Life at a video game magazine office.  Scott Kurtz has created great characters, presented with a crisp drawing style.
The Order of the Stick - D&D humor. If you've never played, you might miss half the jokes. Stunningly simplistic art.
Achewood - Named after an ingredient that was used to make a drink similar to absinthe.
Diesel Sweeties - I think "Love and Pixels" sums it up.
Mac Hall - Art... so... good... gah!
Freefall - Sam is a Cthuloid alien without morals, Florence is a caring and intelligent canid, and Helix is an innocent robot.  Sci-fi, furry, and rated G.
Bunny - Always relevant. One panel, one character, infinite google searches.
Irregular Webcomic - Lego implementation. Gaming & other humor.
Little Gamers - The South Park of online comics.  Crass Kawaii.
General Protection Fault - Jeff Darlington doesn't draw pupils.  Consistent plot line, geek humor, updated like clockwork.
Kevin and Kell - Only an inch south of "The Family Circus".  Still, it's a cornerstone of online comics.
Real Life - Why do they always have their hands in their pockets?  'Cause Greg Dean can't draw hands.  Neither can I.  Video games, computers, medieval weapons.
Nodwick - Gaming humor. There's no end to making fun of the D&D universe.
Dork Tower - Gaminggaminggaminggaming.  Just like crack: you start, but you never stop.
Schlock Mercenary - Take every sci-fi cliche, ever.  Stick your arms out, inflate your chest, act suprised.  Now you're Schlocked.
Ctrl-Alt-Del - Video game comic with violence and okay art.
Goats - Mr. Rosenberg went to Cornell.  That starts to explain his humor.
10K Commotion - A whole strip about DDR! Great style.
White Ninja - Kinda gross. If you like toenail clippings, you might like this.
Red Meat - "I hate you, Milkman Dan."
Kung Fool - Sassy Flash animation.  Killer art.  I'd like a side of plot, though.
MNFTIU - Tycho said it: he "carves" a comic out of profanity.  
Skinny Panda - Pencil sketch style. Reminds me of Bloom County. 
When I Am King - An ancient tale of royalty, love, adventure, loss, and beastiality.  No words, great style.  No more updates, but worth the archive.
Absurd Notions - Intelligence is a curse.  Watch the fallout. Sporadic updates.
Sluggy Freelance - Sluggy is a grandfather of online comics.  Classic geek humor.  Sci-fi, fantasy, horror, satire, violence, and Bun-Bun.
User Friendly - Illiad lays down embarrassingly accurate geek humor, sometimes borrowing *too* heavily from Berke Breathed.  Your one stop shopping for Linux humor.
MegaTokyo - Piro lays down the finest manga-style art this side of Hawaii.  Largo, in the style of Gabe, has "L33T N3KKID SKILLZ".  Anime, video game, computer humor.
Funny Farm - The odd lives of anthropomorphic animal housemates.  Anime, computer, and general weirdness. 
Nukees - Life in the Nuclear Engineering department.  Yeah.  *I* think it's funny.
Sinfest - Pimp humor abounds.  Excellent art, questionable storylines.  Slick does bear an uncanny resemblence to Calvin.  After I read Sinfest, I wash my hands.
The Class Menagerie - Anthropomorphic college dormitory life.  Adorable drawing style, familiar characters. No more updates, though. :(