A new update coming April 18 to rebalance weapons and such:
Read about it at Gamespot
Cool - there's also a new "episode" of Half Life 2 coming out called Aftermath.
There is also another expansion planned called "Lost Coast" which is targeted only at the very high end computers / graphics cards.
Two new free Halo maps are available now. Don't forget to pull them down before Xbox night. There are also 2 new 'pay' maps available.