Bungie: Be a Hero
Bungie has stepped up and and is offering a $100 for every thousand hero's that play Halo3 or ODST "any time next from 00:01AM PST Wednesday, 1/20, through 23:59PM PST Thursday 1/21." I know it's a lot of players to get a $100 bucks but this is a free way for those of us who have the game to contribute in addition to any other contributions we may make.
To be honest I haven't played this since ODST came out, but its on my shelf so this is an option. So if all I have to do is play one round from the looks of it, between the times above, that is something I can definitly do.
They have some specific requirement, your logo/symbol has to be changed just follow the link for directions.
As they say get a group together take a picture, tag it "HERO" and you have a chance to win a T-Shirt.
Here's the link - Bungie: Be a Hero!
Edit: Sorry, I didn't mention this is for the relief effort in Haiti.