The chances of a Covenant invasion happening at Horizon Japan are small...but never zero. Just in case, unlock the Master Chief Outfit available this week.
There's a pretty cool restoration project for the leaked code of Half-Life 2: Episode 3!
I made a small test map for it. It has the Icegun and the Blob. It's pretty fun to mess with!
There is much more than this but personally I find these two things the most fascinating.