Over at Open Source Design we have a job board with various job postings from free & open source projects. 
No matter if logo design, interaction design or website, and whether it’s volunteer or paid – you can find a place to contribute.
And if you do have a project of your own where you need design help, you can of course submit it there too. 
Also join our own Open Source Design Discourse forum if you are interested in getting involved more. 