I bought a LaCrosse 8610 for this exact purpose. It has an RS232 connection and there is Linux software for it.

The problem is that it works fine for about a day and then gets stuck and fails to update data. It only does this when it's being read from by the Linux software. I've spent many hours trying to figure out the problem to no avail. (The serial protocol is insane. It's like a munged direct memory map.)

I haven't found anything else cheap and ready-made, though I'm sure it would be easy enough to make your own solution with a Dallas 1-wire thermometer.