I was planning on coding my own from the ground up in Blitzmax - I still might. However, Novashell, which can be found at the link Gabriel provided looks quite interesting. I mean, look at this feature list!
Multi-platform support using the Clanlib library
A*Star pathfinding
Hierarchal goal based AI system enables entities to think and act on events and changing situations in a flexible manner
Advanced 2D Physics
Lua scripting (used in games like Baldur's Gate, FarCry, and Warcraft 3)
Free form pixel accurate sprite patch editing as well as emulated conventional tile editing
Unlimited zooming in and out, can show entire worlds on the screen in real time
Support for irregularly shaped maps, dead space never wastes memory
Map dimensions can flexibly change during play, new areas can be added anywhere at any time
Robust automatic save/load allows a fully persistent world
Built in world editor with unlimited brush sizes, multiple undo, cut and paste of any size, even the entire map
Virtual palette map concept, create a map or maps with the tiles you use most to act as a palette to work from
Mini-map system
"Fast forward" game speed feature, hold down a key to speed things up
Visual Profile system makes it easy to apply new looks and animations to entities without modifying the underlying game logic
Parallax scrolling
Supports any resolution in windowed or fullscreen, scaling, rotation, tinting, and mirroring of entities.
Advanced audio system supporting music crossfading, positional sounds, and effect filtering such as echo
Real-time "Smart Shadow" generation based on collision information
Create stand-alone games
Comprehensive game modding and modding of a mod support
Multi-platform support using the Clanlib library
A*Star pathfinding
Hierarchal goal based AI system enables entities to think and act on events and changing situations in a flexible manner
Advanced 2D Physics
Lua scripting (used in games like Baldur's Gate, FarCry, and Warcraft 3)
Free form pixel accurate sprite patch editing as well as emulated conventional tile editing
Unlimited zooming in and out, can show entire worlds on the screen in real time
Support for irregularly shaped maps, dead space never wastes memory
Map dimensions can flexibly change during play, new areas can be added anywhere at any time
Robust automatic save/load allows a fully persistent world
Built in world editor with unlimited brush sizes, multiple undo, cut and paste of any size, even the entire map
Virtual palette map concept, create a map or maps with the tiles you use most to act as a palette to work from
Mini-map system
"Fast forward" game speed feature, hold down a key to speed things up
Visual Profile system makes it easy to apply new looks and animations to entities without modifying the underlying game logic
Parallax scrolling
Supports any resolution in windowed or fullscreen, scaling, rotation, tinting, and mirroring of entities.
Advanced audio system supporting music crossfading, positional sounds, and effect filtering such as echo
Real-time "Smart Shadow" generation based on collision information
Create stand-alone games
Comprehensive game modding and modding of a mod support