I bought one of these off ebay for $12.
I had a great deal of fun last night, turning a 120mm pc fan on & off from about 20 metres away. Apparently they are good for about 80m. A friend of mine has a 4 channel switch on his car to run various lights and gadgets. I was going to do something similar but it really got me thinking, you could probably do some really cool stuff with one of these.
I also grabbed a 12v temp controlled relay for an intercooler fan on my vehicle (for low speed high rpm), the system seems to work well