thermostat.
simply the most important peice of equipment after a thermometer.
If it is to regulate a lamp you will need to use a proportional one such as a helix or a herpstat.
For heat matts etc you can use the on/off types like ranco or johnson (sp?).
It's one area where I don't recomend skimping as it will last for a long time, can be transfered between enclosures - and if it goes wrong can kill your pet.
Rheostats are ok if you check and adjust frequently and don't have massive temp swings but nothing beats the reliability of a thermostat dialed in correctly.
dr del