I found myself wanting a dedicated power monitor to use while developing battery-powered application circuits, so I built one using the MSP430G2402.
http://code.google.com/p/powerscope/
This device has already helped me identify and eliminate about 40% of the power usage of a project in-progress.
Right now, I just have the code as a zip file on the download tab, but if anyone wants to develop additional features, I can check the code into Subversion and create some branches.
The board can be ordered for about $12 (for 3 boards) by uploading the Eagle .brd file to http://www.oshpark.com. I don’t make any money from oshpark.