GHI Electronics manufactures a range of .NET MF boards including several Arduino-compatible ones. The most commonly seen ones are the Panda and Panda II. The Panda is a nice $35 board, similar in capabilities to the Netduino. The Panda II for only $40 adds in SD card support, analog audio playback, and a 40 pin header which provides access to a lot of extra IOs. While neither of these boards include on-board ethernet, they do offer a shield which adds that and their e-block ports for $35. If selecting between the Panda and Panda II, I'd personally pick the Panda II every time unless the extra space taken up by the 40 pin header would get in the way. GHI also has the more expensive FEZ Domino and a slim and trim FEZ Mini for $40.