You mention gumstix(>$100), but no the robot-brain focused robostix product from the same company, which costs $39 and more or less blows the socks off the stamp. I voted "other" cos I'd go with robostix. Then if you need to step up brain power later (and your budget goes up beyond $60 obviously), you can stack a gumstix on top of the robostix and add a mega-brain.

i dont know much about them, but ive been hearing lots about the arduino.

its like a stamp, but much cheaper.. (much much much cheaper..)

they sell a micro processor "arm-mite evaluation board" with a cable, a processing board and cd software. i bought one for 66 after shipping charges and it seems to work good. it has 24 i/o pins and 8 are 10 bit a/d. there are a few bad things about it however. first off it doesn't actually have an ic microcontroller such as the stamp. it is fully integrated into a larger board so you cannot upgrade to a larger or more powerful microcontroller unit later. it also doesn't come with its own power supply but i took my old junk linksys router power supply and hooked it up just fine. i also bought a ic connector for $$1.30 and cut it up so i could solder it to the 24 i/o pins so i could easily and cheaply connect some small gauge wire into the controller with ease instead of having to solder it directly into my project. it can handle basic code up to 10million lines + per second or it has a c compiler too you can use but i haven't tried either yet.
it connected to my computer in minutes and i uploaded a very simple program to it in the first minute i had it running that just displayed on the comp screen "hi mom". they suggested to print something to the screen to see if the controller actually was hooked up correctly. if i have any major problems i'll try to report later