This has been a very recent topic at our company that was bootstrapped early this year and now is humming along nicely. We are formally implementing an Open Source Friday each month (would love to do every Fri, but company not doing quite that well). We're suggesting three avenues, but we're not limiting it to these:
1. Contributing design (we're trained designers that also develop) work to projects we actively and implicitly use.
2. Open sourcing our own novel technologies and working on documentation, issues, and community.
3. IF (there better be a good reason) we have to work on this Friday, half of the revenue for the day will be donated to supporting Open Source Software.
We'd also love to hear ideas from others on ways designers could help out the Open Source community.