DAC's own BryanR has created a multiplayer mod for Fallout 3. Now, you'll be able to take out those pesky radscorpions in teams! While still in the early stages of testing and implementation, the mod promises to dramatically improve the replay-value of Fallout 3. Check out the test video below:
Here's the rundown from BryanR:
Basically I have a server and client built just for the purpose of multiplayer Fallout 3. The client program I made reads from a special .esp file which I am using to do my dirtywork of collecting information on players. This information is forwarded to the server. The server then forwards it to the other clients, and then the script variables are written to to reflect changes.
DAC will keep you updated on any new developments.