[cottonseed]

Documentation! See this recent thread for example:

https://news.ycombinator.com/item?id=13437252

Or generally anything that helps user experience: documentation, improving web site/documentation design, testing, finding bugs and making high quality bug reports, evangelizing the project to users who need it but aren't aware, becoming a expert user of the project (which will almost certainly lead to bug reports, suggestions for improvements to features and documentation, etc.) and triaging tickets, answering questions in the support forum, etc.