Quantity   Component name
1 × Raspberry Pi, ~$35
1 × 5v Opto-isolated Controlled Mains Relay, Under $10 from eBay
1 × Heavy duty electrical box from Home Depot, $11 (I think)
1 × Single electrical wall plug, Home Depot, $5
1 × 1A 5V USB power supply, Lying around the house
3 × Waterproof DS18B20 (At least 1, I'm using three) http://www.adafruit.com/product/381
1 × Electrically Powered Slow Cooker It needs to be a cheap one. No digital controls (unless you want to hack it), with a mechanical power switch, no thermal inhibitors when on the high setting.