Participants to the CONET SENIOT Summer School held in Bertinoro, Italy, this summer had the opportunity to attend an interesting hands-on session with basic exercises with the Contiki Operating System for WSN. Material from the course is available online, you could download the slides from the presentation and session notes. The material is self consistent so you could try it out too!
Contiki, developed mostly at SICS since 2003, has reached a level of stability that is interesting to explore further and it also has a growing community of users. Earlier this month, two new ports were made avaialbe: Crosbow MicaZ and Sensinode, besides other platforms already supported.
Everything Contiki available here