Careful what you are buying when you pick up Red Orchestra. Red Orchestra used to be and probably still is an add-on for UT2004. They garnered one of the top mods in a contest for UT2004 modding. I've played quite a bit with it at one point and had some fun.
If you want Red Orchestra for free, you have to have UT2004. You can get the mod here:
http://www.fileshack.com/browse.x?cat=2561
Another good UT2004 mod is Alien Swarm (If you like the old movie aliens). It transforms UT into a top down tactical shooter that is full on action:
http://www.fileshack.com/browse.x?cat=2250
Some info on Alien Swarm can be found here:
http://www.blackcatgames.com/swarm/info