Much of the web talks about Yet Another GNU ARM Toolchain, but I couldn't get yagarto-4.4.2 to install on MacOS 10.6.
Happily James Snyder has packaged them into a convenient Makefile:
git clone http://github.com/jsnyder/arm-eabi-toolchain.git cd arm-eabi-toolchain sudo make install-deps <edit Makefile to change install dir, which should be on the PATH> make cross-install
There are several different ARM compilers here, though at the time of writing (May 2010) James Synder's versions seem to be newer.
You can download the source but it's easier to get it from Mac Ports:
sudo port install openocd "The documentation":http://openocd.berlios.de/web/?page_id=54 is helpful.