I would like to introduce a project I've been working on for the past year or so. The Propeller ASC. It is designed to be plug-in compatible with most existing Arduino shields. It adheres strictly to the mounting hole and connector layout of the Arduino, so it will fit existing enclosures as well. It is programmed in Paralax's SPIN language as well as native assembly (PASM). There is also 3rd party language support for many others incl ANSI C. It is not compatible with Arduino sketches (yet). The most unique feature of the Propeller is it's ability to generate simple video (VGA & Composite) using just a few resistors.
I am not affiliated with Parallax, I am making these myself. More info can be found here
If you don't know what the Propeller (P8X32A) uC is, I suggest skimming though this comparison/debate
on the Parallax forum regarding the Propeller, ARM and XMOS. It does a good job at explaining how a chip with no interrupts and no hardware peripherals can be just as versatile and easy(er) to use.