Natural Selection (Half Life 1) Natural Selection Gameplay - Sorry for Bad Quality, Taken off my old computer.
Aliens versus Marines (Humans)
Team Based Game Modes (Commander Controls Resources and Upgrades) and base destroy where each team attacks the others Command Center (Marines) or Hive (Aliens).
Players gain XP for doing damage and kills. Each level can unlock new gear and abilities. Per round. Not constant thorough out all servers but its fairly easy to level up.
Playing also for 4 some years alongside The Specialists.
Plenty of active servers.
Very Fun Game