The STM32 Value Line Discovery board is a terrific platform for exploring the STM32. It has a USB debugger connection. There are some good links on the web to get you started:
Much has been made of the fact that the standard tools and demo programs are windows oriented. It turns out that Linux and OS X are fine development platforms for this board if you're willing to run a single binary with a Windows (real or virtual) machine to act as a proxy for the ST Link. Furthermore, all of the ST example programs build quite nicely in a Unix environment.
The sidebar provides links to pages describing how to make this work on OS X (linux is easier !).