Rebel Strike Force is a mod changing the regular HL2DM setup into a more on teamplay based game. A player can choose from 8 different classes to fill in a certain role in his team. There currently is only one gametype called "Objective". The idea is that you and your team mates try to complete a given mission while the other team tries to prevent that from happening.
Here are some screenshots showing one of the maps in RSF and also some game features.
Really cool.