As with the others, it would be awesome if people want to contribute to this so we can keep this nice and complete. If there's anything you want explained, also comment here and our crack team will get on the case.
(Last Updated: Feb 08 2010)
GJ - Good Job!
otsu / 乙 - A rather more formal Good Job!, i.e. 'Thank you very much for the hard work' or similar.
wwwwww - Laughter.
2828 - Grinning or smiling - less laughter and more 'yesssss' than www.
4649 - yoroshiku, 'pleased to meet you'.
ktkr - kitakore, 'It's here!' or 'It's arrived!'; think of it as kind of reverent, 'the time is now!!' sort of stuff.
(キタ━━━━━━(゚∀゚)━━━━━━ !!!!!) - kita!!, a more excited version of ktkr.
wktk - wakuteka, excitement or eager anticipation.
dkdk - dokidoki, excitement with a side order of crushing.
kwsk / くわしく - 'MOAR!'
zawa / ... ざわ... - the anxious pause before something shocking
(ry - ryaku, used to denote cutting short an obvious, comedic or meme reply. e.g. 'Ceiling Cat is(ry'.
orz / OTL / any number of variations - despair (it's a guy slumped on all fours)
sage / age - (set) a good example, for indicating condemnation or support respectively.
うp - 'upload'.
p - 'producer'.
しょくにん / shokunin - 'craftsman', usually used as an exclamation when something is particularly well-made.
神 / kami - 'god', similarly to 'craftsman'.