I've picked up a number of games, both from this thread and just poking around, but I spent about an hour last night playing Deathtrap, and it is pretty great.
FWIW, I really enjoy tower defense games, and this is one of those, albeit the type that allows you to run around and supplement your towers abilities. But instead of being first person or third person over the shoulder view, it is more like a MOBA view, and even though I'm not much of a MOBA guy, I really like it. I could see spending a lot of time in this game.
You don't have much flexibility in the way of traps, there are spots where specific traps go, and you don't have many choices (eventually you can unlock variations and upgrades), but I find the lava and acid spray traps very satisfying. Also, you get gear to level up your guy, and that's been oddly satisfying too.
If you like tower defense style games, there's worse ways to spend $5.