Lol.......its been a long time since I used it, I am not sure how popular it still is.
Basically xboxconnect has a bunch of xbox players on it (mostly halo) and you can join up with them. It uses the system link option to fool the game into thinking its system linking but you are really playing online. It can be laggy, but also fun. If you have questions, post em up!
heres kinda a topic about it from a while ago: http://www.egameaddiction.com/forums/index.php?topic=108.0