What options are there for GLSL shader development on OS X?

There's a tool called Shader Maker which is based on Qt. Lumina is also open source.

One of the best GLSL editors I've encountered is ATI's RenderMonkey, but there is sadly no Mac OS X version.

Quartz Composer (included with the OS X development tools) has some GLSL editing functionality but it's probably not what you want. This is an example of something developed using the tool.