I'm (obviously?) a big supporter of open source software. I've been contributing back to open source projects -- at least with patches, package building, etc. -- for years, and using it for even longer. As the Operations Manager at the Oregon State University Open Source Lab, I have the opportunity to work closely with many open source projects, and also oversee student employees while they work on various projects both contributing code and helping maintain the hosting infrastructure at the OSL. From my experience, open source experience has given our student employees an enormous boost when looking for their first jobs out of college.
What is it that employers see as a benefit of hiring a student with open source experience? I think that question has many answers that depend on the employer, but there are some general answers that are likely common among most employers. Here are some reasons that I think being involved with one or more open source projects makes you a more desirable employee: