Just for the record and to save the links for myself: Maple from LeafLabas. "Maple is a 72 MHz 32 bits ARM Cortex M3 chip [...] built from open source components and a programming environment that is intuitively easy to use."
Maple Leaf is built on the same form factor as the Arduino boards, so Arduino shields will fit. It has 20 kB RAM and 128 kB Flash, and is only $50 (the Arduino Uno is $30). Also interesting is the Maple Native, which brings 64KB RAM and 512KB Flash.