Need a good game to buy in summer sale

You should buy Ace of Spades, since my tastes aren't the same as yours.

If you don't already have it, I suggest keeping an eye out for Payday 2, Killing Floor 1 and 2, Sanctum 2, Natural Selection 2, Orcs must die 1 + 2, Just cause 2 (multiplayer add on is free).

Try searching on steam with the tags similar to what you like. Easy to find stuff then or see what your friends are also playing.

Left 4 Dead 2 maybe? If you like mmorpgs you might try Secret world.
If you'd like coop with a friend instead then you might want to try Saints row 3 and 4. Borderlands 2 is also good for coop.

red orchestra 2 and rising storm.

If you like something more tactical than maybe arma. Project Reality Standalone also came out so you should give that a try.

Killing Floor 2

Quake 3

Insurgency.