You're out looking for a job, and you want an edge over the rest of the candidates out there. Your experience in open source should count for something, right? It just might, but the key is how you sell it to the person who reads your resume, and to the interviewer in an interview.
First, think of each project as a freelance job that you've worked on. Just as different freelance gigs have varying sizes and scope, so too does each project to which you contribute. The key is to not lump all your projects under one "open source work" heading.
Explain in your resume the contributions you've given to each project. Don't assume that someone will understand what your project is, or immediately grasp the importance of what you've done. For example, on my resume I might have:
Perl programming language (www.perl.org)
Created the prove command line testing tool. prove allows the programmer to interactively and selectively run tests in a test suite without a Makefile, making test-first development much easier. I wrote prove in 2005, and it was immediately embraced by the Perl testing community. It has been part of the core Perl distribution since 2006.
As with anything you put on your resume, explain what you did and why it was good that you did it. The only difference between project work and a "real" company is that instead of explaining the value to the company, you're explaining the value to the project or to the users.
Wags familiar with prove may say "But all you did was write a couple hundred lines of code around the standard Test::Harness module." The key to someone looking to hire me isn't what I did, but why I did it, and that I took the initiative to do it at all. I saw a need for a tool, created it, and released it to the world, to much appreciation.
So what have you done to contribute to help open source projects? It doesn't have to be as big as a deal as you might think. Submitted a code patch? Explain the bug, how you fixed it, and what you did to get the patch into the system.
As with any project, make sure you explain what the project if there's any chance someone reading might not be familiar with it.
(Thanks to Esther Schindler for asking for comments in her article "What To Include In Your Open Source Resume", which prompted this posting.)