I second Americas Army.  Its a wonderful game and the community is pretty good.  The style of play is slower than CS, which leads to more mature players to begin with.  There is pretty tight control over cheating on the official servers, so its pretty good.  Clan matches with CS were good, but too often ladders were filled with cheaters.  The Rainbow Six community was always pretty good, and most of those guys should be moving along to Raven Shield when that comes out.