The Boxhead series is an old internet favorite, and the relatively newest iteration, Boxhead More Rooms, is so far the best of the bunch, taking the same addictive zombie killing madness and opening up a lot more room configurations and layouts to puzzle over.
It's a fairly standard top down shooter, but with a good variety of weapons and traps which force you to think strategically, or die.
Like all good zombie games, you most often get brought down by arrogance or a simple mistake- which leads to lots of replays to refine your strategy and prove yourself to...yourself. The online leaderboards also help.
You can play Boxhead More Rooms here.