The RockStar GTA IV site is a work in progress with a little more being added/leaked all the time. Now the Multiplayer section is up and running, there is still much to be added but atleast they have some info now. Check the link for screens and discriptions of game modes.
Do you like races? How about trying out the Multiplayer Race mode where the entire map can be used. And choices can be made such as Car, Helicopter, Motorcycle or Boat races on player created courses throughout Liberty City. Click on the R* link below! Then go to the bottom of the R* page and click on "MULTIPLAYER"