Ok, there are a ton of these, and they've been shared between geeks since before it was called 'the Internet' in Vice-President Gore's feeble imagination. But I'd like to get a comprehensive list of them, so let's start here!

PEBKAC: (Problem Exists Between Keyboard and Chair)

I D 10 T: (This is a spoken joke. You say "eye dee ten tee", but when you write it out "ID10T" it obviously becomes 'idiot'.)

STFW: Search The F$cking Web

RTFM: Read The F$cking Manual (Man page, for us UNIX purists)