Try Voltcraft TCM 220 Digital Temperature Switching Module -30 to +70 °C. Has 2 channels with 2 probes. Has LCD, alarm treshold limits (too low and too high), local buzzer, serial port etc.
http://www.conrad.com/ce/en/product/126609/Voltcraft-TCM-220-Digital-Temperature-Switching-Module-30-to-70-C
Works with two AA batteries, their lifetime is 3 years+ (mine are running for 4 years now, no low battery signs). It's got a serial output, which can be simply polled from a PC.
I wrote a Munin-plugin for this some time ago, works flawlessly in my room, one probe close to the floor, the other close to the ceilig, gaph plotting both:
http://munin-monitoring.org/browser/munin-contrib/plugins/sensors/voltcraft_tcm220_
I also like that it has an LCD, so whenever I go down to the room I can check the current values just looking at it (switching between the channels is done with a push-button).