Released last year, NS2 is now 90% off on Steam.
Natural Selection 2 - $24.99 USD $2.49 USD
Natural Selection 2 4-pack (can only be bought if you don't already own the game) - $74.99 USD $7.49 USD
Natural Selection 2 Deluxe Edition - $39.99 USD $3.99 USD
Natural Selection 2 Deluxe Edition Upgrade - $14.99 USD $1.49 USD
Set aside some time for this game, it takes a while to get comfy with it. I tried when I didn't have a huge amount of free time and I'm still getting to grips with it. Fun though, plenty of active servers, and mostly friendly players. Just be sure to have a headset or something and at the very least listen to what your team mates are telling you to do, this game is so heavily reliant on working together.