Zdefense was given away a few days back on Amazon, and it's basically HexDefense with an editor thrown in, and difficulty turned WAY up, and a few twists.
The actual combat is standard fare... "creeps" emerge from entrance hex, and goes for the exit hex, via whatever efficient way they see. Your job is to kill them and prevent them from getting through, using whatever turrets available. Turret choices are your standard list: cannon, laser, missile, shockwave, slow. Each can be upgraded umpteenth times. Kill creeps to get money for upgrades.
This game throws in a few more twists
Mountains: a tower on a mountain hex have vastly improved range
No-build hexes: you can't build on those, but creeps and move on them
Warp-gates: creeps enter a warp gate will instantly emerge from the matching warp gate at a different sector of the map
Tunnels: creeps that enter the tunnel will stay inside and move until it gets to tunnel exit (and can be shot at in the meanwhile)
And as said, it even has a map editor, and ability to take screenshots to celebrate your winning moment (and share them with friends). The maps can then be shared as well.
It also features 3 levels of difficulty, and even on easy, some of these maps are quite hard.
At $1.99 regular price, this is not a bad way to spend it, albeit a bit derivative.