I used to do temperature sensing on my BBC Micro. They were neat in that they had an analogue joystick port and you could put a thermistor straight on it and you were away.

Anyway, here's three different ideas:

http://www.ugcs.caltech.edu/~dstaff/sysmeter.html
http://www.zejn.si/~natan/electro/tempmeter/
http://www.cesko.host.sk/GamePortTemp_eng.htm (same as my BBC Micro experiments)

Gareth