Ask HN: How valuable are open source contributions in/on your portfolio/CV?

I have way too much free time on my hands (the weekends) but no ideas for side projects. I could pick a few projects on Github in my language of choice and just start fixing bugs and when time comes doing future requests et cetera. How valuable are these contributions for future employers or just as an "experience" indicator?Better, worse or on-par with doing actual side projects? This is because I am currently working at a company writing _only_ proprietary software so future employers (if I ever have to switch) have to believe me on my word when I write on my CV or tell them that I'm a good programmer.