Rapid Prototyping for Microcontrollers: mbed.org

mbed is a free service that let you prototyping with a microcontroller board. To give more information here, the best is perhaps to copy the following paragraph from their web site:

What mbed is good for!

mbed is designed for quick experimentation and iteration, 
where the focus is trying something out rather than optimising 
a solution. And that means it is good for anyone from 
professional embedded engineers to people with expertise 
outside embedded exploring it for the first time. Think of it as a pencil 
and paper, compared to a CAD program (the professional tools).