The Netduino is a .net powered hardware platform. Basing it on .net makes the transition easier for those of you who are used to working in the .net domain. It relies on free (as in free beer) tools, but is not 100% open source kosher. For those of you who wants to get started, check out the Coding4Fun blog.
The Make site both sells and introduces the project. For those of you who like to take hacking a bit further, the platform uses an AT91SAM7X512. This little chip apparently is capable of running FreeRTOS.