I've used the Keil IDE, which is free for small projects. Takes some getting used to but great debugger and easy to get started. There's also free Eclipse/gcc

I've used the Keil IDE, which is free for small projects. Takes some getting used to but great debugger and easy to get started. There's also free Eclipse/gcc toolchains so if you're already used to Eclipse that's an easy way to get started. All the newer ST demo boards have on-board JTAG debuggers so they're a little bigger but convenient since you just need a USB cable. External JTAG debuggers are pretty cheap and Keil (ARM) has a super-nice trace probe if you're willing to spend a little more.