Keg - um, the guy in his sig. With a green INF rag.
Donnelizer - a fat dancing Blackmage
Browncow - short, stocky guy with a mowhawk and a massive spiked black leather jockstrap. And a horned helm.
Yurch - either Captain America with a pimp hat OR Barney the Dinosaur with a pimp hat OR an Orc with a pimp hat. Or just the pimp hat.
Mad Dog - punk guy in his avatar
CrappyChan - Mr. Hankey with martial arts and an accent
DarkBls - a dolphin with huge nads and a FAMAS
Beerbaron - some crazy ugly black woman who eats babies
Warren - the King from an INF Tarot deck
Keiichi - Tenchi
Fur - Skinny guy, something like a younger, Hispanic Nietzsche without the moustache.
TitanBlue - weird German guy who does this
a lot
McMuffin - tall, lanky guy with a huge broadsword and a kilt. Maybe a Jesusesque crown of thorns as well. (I don't know where I get these images)
Deeznuts - dark, nebulous, geometrically impossible figure. Think Lovecraft meets GWAR.
DoomFreak - the cacodemon in his avatar and the INF guy in his sig superimposed to form a single entity. Uh, not morphed or merged, mind you, just... superimposed, as if on a sixth dimensional axis, kinda hard to explain
what I actually see...
El Diablo - little guy in dusty brown robes and glowing red eyes, smoking a bud.
Thrash - really straight-looking Christian guy with dark curly hair and a guitar.
Rogue - Charlie Chaplin in a Soviet uniform.
Stormcaller - tiny, silhouetted figure off in the distance in my sights on Atoll
Phatcat - Anime catgirl sans pantalons (IT'S THAT DAMN SIG)
Zund - Nyarlathotep with a gun
Profe - winged swine
Densraw - Chibi Sauron with down syndrome
Lifesbane - teh nekkid midget
MadWoffen - a boar; I can't quite make out what that thing in his avatar is, I keep looking at it and it reminds me of a cartoon boar...
Max Sterling - like his Macross namesake, but French and with brown hair
Pacman - chain-smoking, short-tempered pale guy with a pink wig