This weather monitor can track pressure, temperature, and humidity and last several months on a pack of AA batteries. It has a PIC18f452 to control all the bits, as well as some fancy power management to get the most out of the batteries as possible. The screen displays the highest and lowest temperatures of the last 42 hours as well as having options to display all other sensor input, updated every minute or so. You can get the full schematic and parts list on the project site.
[via YourITronics]