The Creeper World series is great. Most maps can be cleared at your own pace (once you figure out how to survive the start) but it's possible to win quickly with optimal play, just check the leaderboards. I tend to play somewhere in between, it's relaxing.
I've been hooked on the third installment since I got it yesterday. I recommend everyone else play the demo for 1, and maybe the random maps here (update daily):
http://www.kongregate.com/games/whiteboardwar/creeper-world-evermoreI think the demos for #1 and #2 are still on Kongregate also. The demo for #3 can be downloaded from the developer's website. You can also just buy the game there to get a non-steam download.
The second game is a side-view instead of a top-down view, and it's interesting in it's own right, but #3 goes back to the top-down style. I would not recommend playing #3 until you've gotten the mechanics from the tutorial in #1. #3 adds a lot of new features which are meant for people, like me, who were hungry for more.
Edit: They also greatly improved the graphics for #3, and it's more efficient. Probably because it isn't AIR anymore. In #2 I would get lag on the bigger maps when I spammed too many units, just from the number of entities.