Here is some links that I found good and interesting.
Level Design
A good place for Level design is Reaching Perfection and The Forge Lessons
Gametype Updates
- At this Gametype fix/improvements thread you can see what Gametypes have been Released and changed.
New Gametypes files
- At T.U.R.F.S. (The Unofficial Reach File Share). This is a great resource for getting the latest Files from matchmaking, with out the need to play matchmaking
- at this Matchmaking repository (that is in the works)
- my File share for key files that have changed or released after Reach shipped in my File Set.