Arduino. The possibilities are endless. I have made many robotic and other projects using these boards. Also have a look at something made by Digilent called Chipkit UNO 32. Its 32bit and has much more IO lines compared to arduino and the best is its pretty much arduino compatible coding etc. In other words you can upload your arduino sketches into these guys. I still use the AVR based arduino environment but normaly take the chips off board and make up PC boards for my projects.
If you ever need some help just ask :-) If I where to choose between the UNO and MEGA id go with Mega.