Besides what was already mentioned (nantucket, phrosty, skierman, hot girl thread, robe & wizard hat etc.)
-There was the HUGE hype and hysteria on some random date like 10/21.. people were going crazy, nobody had a clue, and mass amounts of conspiracy theories ensued. (If you remember what the actual date was. let me know.. I'm running a blank)
-Breaking story of a prominent NS member who commited some crime, was on the run, and crossed the border. (I think it was NinemillaKilla ? or skipimp?) He used to be on NS everyday.. and went missing for weeks, right about when the story came out.. so it made it that much more believable.
-Bryan Gallant - battles. hate threads. photoshop contests. and infamous ski video of him that plagued the whole internet.
http://youtube.com/watch?v=Y6d-bSDDM2I
-Snowblader threads. And the snowbladers that 'jumped' Chad's Gap.
I'm surprised no one mentioned Pikachu. When Pikachu first appeared on ns it was hilarious..
-NS went offline. Server's crashed. (happened a few times. people went berserk)
NS Spell Check.
-Weiner. that guy had some hilarious threads.