I used Flixel. I had to add a lot to get most of the game mechanics in a reasonable way. Flixel provides the basic platforming phyiscs, but I had to add things like variable jumps, dialog screens, pickups, enemies, inventory, etc.

The world is actually a collection of text files in a format Flixel uses to make the basic physics. Each level has three files: background, widgets, and enemies. I modified the file parser so I could spawn enemies, doors, and triggers easily.

We used a free level editor to make the text files.