The TuxCap Games Framework is a GNU/Linux and Mac OSX port of the PopCap Games Framework used for 2D game development. It comes with PyCap Python bindings, a fast 2D physics engine, a particle engine, widgets and many documented examples.
Cross platform 2D game creation framework with Python bindings
FIFE is a cross platform 2D game creation framework written in C++ with Python bindings. It's designed to be flexible enough to support a wide variety of 2D game types but specializes in 2D isometric type views.
The main purpose of the Moonlight Engine is to be a general 2D game engine geared towards tile-based games, and RPGs specifically. Support for online games is also planned.
NiL is a modular multi-purpose gaming engine including a multiplayer worm game a little like quake, except it's 2d and a little like worms except it's realtime.