I’ve always personally filed this under the “if I ever need one of these, I’m gonna freakin’ move so that I don’t” category, but I’m still fascinated by them. Block heaters, according to Wikipedia, are generally installed via a freeze plug opening in the block, though some sources use heated dipsticks. The basic idea here is to keep the block — metal, fluids, and all — at a reasonable temperature when the weather outside, well, isn’t.
While I’ve never personally tried it, installing your own block heater doesn’t appear to be that difficult of a task. Froogle turned up a number of heaters ranging from $25 to $200, most of which install via the freeze plug method.
Removing a freeze plug is a relatively easy task: simply drill a couple of holes in it — slowly to prevent adding metal shavings to the inside of the block — then attach a wire or string to the plug and knock it free. (The wire helps if you accidentally knock it into the block. Using the wire is way easier than disassembling the engine. Really.) Tap the new one — or the heater — into place carefully, then check the seal.
Of course, advice from our northern friends is happily accepted.
Block Heaters [Froogle]