ATtiny85 and 2313's to try my hand at [gulp] proper AVR programming. It's too bad that Arduino is going toward the high end with the Due instead of toward the low end with smaller, simpler circuits and chips, but I'm happy to go elsewhere to learn. I could hot-wire the Arduino environment to program an ATtiny, but I'd like to try transitioning to a more fully-featured IDE for future development anyway, so programming an ATtiny will be a good sub-project.
Speaking of "the high end," the $25?! (OK really $50 but still!) Raspberry Pi has been on my radar, maybe for something using some cameras and image analysis-- things an Arduino would struggle with. I'm especially excited lately after hacking around with a $25 WiFi access point and OpenWrt... wow! Amazing how much power can be had for so cheap, and how many how-to's can help you along.
What do you think? Have you left the "safe" space that Arduino provides, and moved to different processors and IDE's? please leave a comment.