Very recently, one of oDesk's product architects based here in Menlo Park posted a job opening on our site to find an additional engineer to add to our growing team. He received a number of candidates and then had to screen and interview to find the top provider to join the team.
He found all of the profile information helpful, and reviewed feedback scores, requested oDesk tests, and looked for personalized cover letters of individuals that seemed truly motivated to do the job.
Once he had narrowed the list down to just a couple candidates. He researched their work history in more detail. He ultimately chose a particular candidate in India largely based on an example of his work. Now, this wasn't a web developer, so he couldn't review a portfolio of websites. He also wasn't able to review prior code because much of it is proprietary for other clients. So, it came down to reviewing prior work on open-source projects.
Why should you care about open-source work?
1) It's completely open. You can review the exact code, revisions, and comments made by the individual contributor.
2) It shows motivation. Open-source work doesn't pay, so if a provider has dedicated some of their personal time to contribute to an open-source project, it demonstrates a high-level of personal motivation.
3) It demonstrates knowledge of relevant technology. For oDesk, and for almost any small tech company these days, open-source technology is critical to business operations. It's important for any engineers to be familiar with a good number of open-source tools.
So, if you are a provider and looking to build out a robust online profile, consider finding an interesting open-source project and contributing however you can. You will always be able to share this work with anyone and demonstrate your performance. Head on over to SourceForge and figure out how you can contribute.