Over the next few weeks I'll be highlighting some tooklits and game engines that people should consider making use of in the code portion of the Liberated Pixel Cup. To kick things off, I'd like to draw your attention to the Frogatto engine, which powers the beautifully rendered Frogatto & Friends.
The Frogatto engine is well optimized and is silky smooth even on slower machines. It is written in C++ and has minimal dependencies (OpenGL, Boost, and SDL), and it runs on "darn near everything", including: Windows, OSX, Linux, iOS, Android, Blackberry's Playbook OS, WebOS, Maemo, and Pandora. And don't be fooled by the fact that Frogatto & Friends is a side scroller -- it's quite capable of handling top-down games as well. (As further evidence of the Frogatto engine's impressive versatility, it's been pointed out to me that it's also been used to write an isometric game, Cube Trains -- see screenshots here and here, and a video.)
Frogatto's game logic is powered by the Frogatto Formula Language (FFL), which you can read about in detail here. What's more, Frogatto comes with its own game editor, which has some live game editing features that have to be seen to be believed (see the video below):
So, if you're trying to decide what engine to base your LPC game on, definitely give Frogatto a look. If you need help getting started, hit up the forum or come talk with the Frogatto dev team on IRC (#frogatto on irc.freenode.net).
Frogatto's game engine documentation can be found here.