1. MonoxideC's Pelican mod on Death Island. This was a huge event in the Halo PC modding community. I remember downloading the map, hopping on Halo PC... and then ramping warthogs off of the top of the Death Island bases and flipping them off the Pelicans. Classic. And actually, this further reminds me... I used to be a moderator at Halomods.com. Wow. That was so damn long ago... :|
2. The Halo 3 Beta. Oh man, when people managed to get custom game lobbies open... good times. Good times indeed.
3. Halo 3's ending. Oh man. I finished Halo 3 just minutes before I had to go get my senior pictures taken for high school. I was a mess. I have no problem admitting that I was bawling my eyes out during the credits, thinking that the chief was actually gone. But the post-credits scene - it was beautiful. It was now tears of happiness.
4. Running through the streets of New Mombasa alone at night in Halo: ODST. The music... ODST has some of the best music in all of Halo. It was just so perfect. When it's dark in the game, and dark in your room, it's magical.
5. Flying the Pelican/Phantom in New Alexandria in Halo Reach. Jumped in a game with Kornman, he told me what to do/what he knew... and despite my terrible totally-does-not-work-for-co-op internet connection, soon enough, we were flying around in Pelicans and Phantoms. So fun.
6. Snipe's Halo LAN in Indianapolis. This was the one and only time I have ever gotten together with HBOers. And boy, was it ever fun. Would have loved to do it again, but... eh, well, I'm kind of on the other side of the world right now, and Halo 4 just isn't fun at all.

*sniff* Good times. :)