Killing Floor 2 for PC for £4.99 at Electronic First. I should stress that I've never purchased anything from this company before, but noticed they've been referenced on this site a lot of times before.
I urge anyone who reads this to do some checks first, just to be sure that the company is above board, etc.
For only £5 (usually £19.99) it's well worth grabbing at this price. Especially as anyone who plays it whilst it's in Early Access gets some free pistol skins and a character skin (Horzine armour).