Cinema Blend Games has a series of pieces by Steve West they're calling their Halo 3 Primer. It covers:

• Multiplayer
• UNSC weapons
• Covenant weapons
• Equipment
• UNSC vehicles
• Covenant vehicles
• Characters
• Story
• Community
• Editions/Accessories

A lot of information there-- most of it not new, but it's nice to have it all in one place.