This article explains how to build your own digital voltmeter based on the INTERSIL CL7107 IC chip. It provides great detail, schematics, pics, a user forum for the project (link at the bottom of the article) and even a couple schematics for a power supply. By changing a resistor, you can vary the measurement range available.
View the article and Build your own digital voltmeter here Be sure to check out the forum and other links for updates before jumping in.
Mouser Electronics is a great resource for the parts listed.
The site, Electronics Lab, has a lot of other great DIY electronic projects too.