If you have Half Life 2 (just HL2, the Eps. are not needed, but it is advised you have them as well), then you should get Stargate: The Last Stand! This is an amazing game based on the Stargate Series (well, somewhat). The game is an offense-defense game with the Tau'ri battling the Goa'uld and vice versa. The project has since been abandoned by Interwave Studios, developer of the game, but some members of the community have taken over in terms of map making! The game itself does poses a few bugs (as mentioned above, never really finished), but the current project is a fun game to play for any Stargate fan.
You can acquire the game here: http://www.moddb.com/mods/stargate-the-last-stand/downloads
Make sure to download and install the "Stargate: The Last Stand 1.1 Client Full" (The third listing).
Currently, there are only two 24/7 servers. The first one, which I recommend, is the SG-X, US based server. Alternatively, you can join the GTG server which is located in Germany.
Contact me if you have any information regarding the game!
--Yolo