Xeno Defense and this week I found Tesla Death Ray, which is almost as addictive, but is clad in a somewhat different costume.
It's a tower defense game in which you have to zap your enemies to oblivion. Much like in Xeno Defense, at the end of each level you get to the store and can use your hard-earned points to purchase upgrades and repair your tower.
Unlike Xeno Defense, you only get to the store if you pass the level. This makes the game much harder, because it means if you've made some wrong choices in terms of upgrades, now you're stuck with a level you simply cannot pass, and need to start the game all over again.
I should at least be able to sell off some of my equipment between retries of the same level, I think. But despite this one flaw, the game manages to be addictive, fun, and fast. I like it, and probably would have finished it if it hadn't been for the upgrade problem.