We use the CodeSourcery package.
CodeSourcery got bout out by Mentor Graphics. They still seem to have a Free/Lite version available:
http://www.mentor.com/embedded-software/sourcery-tools/sourcery-codebench/editions/lite-edition/
The very least you need is a working cross-compiler and cross-linker with C libraries built for the target. The usual tools like nm, gdb, objdump and so on are also useful. That's all we took from the CodeSourcery package, together with a version of "make" that runs under windows. We managed to get the gcc to talk to a USB pod and debug it that way, from the gdb command line.
If you already have a complete working code base (including operation system or event-loop and device drivers) then that's all you need.
If you want a fully featured "integrated development environment" that works like Visual Studio does, then you might be able to use Eclipse, but I've never had any luck with it.
Tom