Most people who use chat/IM programs know what LOL means: Laughing Out Loud (or, if you’re dutch, it means “fun”). LOL
Several people have asked me recently what ROFL means. It means Rolling On (the) Floor Laughing. It’s another one of those net acronyms, but I guess it’s fallen out of popular use (since few people *actually* roll on the floor laughing when they use ROFL). Variations include ROTFL, ROFLOL, ROFLMAO, and my favourite, ROTFLAICGU (Rolling On The Floor Laughing And I Can’t Get Up).
Check out Wikipedia’s article and list of Internet slang. It’s fairly up to date and complete, but it lacks a lot of the oldskool (read: archaic) terms, like TANSTAAFL or DILLIGAFF. For those, you may have to check elsewhere, like the Jargon File (see also the Wiki version).
NOTE: the above links contain some coarse language – view at your own risk.