Very Popular, Good Fun : Customization and Mods

I don't see many posts on the customization of this game, so here it is. CS:S is a great game for you if you're interested in FPS games. Very simple: two teams, terrorist and counter-terrorist. Standard maps have one pre-set game mode: either the terrorists must plant the bomb, or the counter terrorists much rescue the hostages.

The big problem with the game: hackers. I suppose script-kiddies is a better term for them, since they use common aimbots and wallhacks. The good thing is, they usually stay away from mod servers, or custom maps, due to the restrictiveness of wallhacks.

When things get boring as you play standard maps, go play on some custom maps (a starwars map comes to mind), such as maps that have vehicles (can be very fun), or maps that allow you to slide across ramps and do tricks (known as "surfing"). There are also quite a few servers which use zombie mod, or superhero mod. This isn't the complete list of mods, so go check it out for yourself : there's a lot more to it than just killing the opposite team on standard maps.