Mbed is completely free, and mostly open source. The even better news is that Mbed doesn’t actually require any installation at all – development is done in an online, web-hosted IDE – and it’s broadly supported by most major players in the microcontroller marketplace. The Mbed system is an excellent way to get to know a new microcontroller.
The main issue with Mbed is that it has very limited debug support. When using their online IDE, you can’t perform single-stepping or set breakpoints, and that limits you just to print debugging. For many developers, Mbed is a good system to start developing for a new micro, but one you may quickly outgrow.