One easy thing you can do to contribute to OSS, ignoring the maintainer issue, is report issues in a clear, concise, understandable, and friendly manner. One of the issues the freerider problem has created is that users using open source software often fight to work around issues, without ever reporting them. While this isn't super helpful for projects short on maintainer time, at least filing issues helps them understand what is going wrong and potentially fix it. Of course filing an issue and being pompous or rude isn't acceptable, but filing issues that are clear and well-documented is a great contribution!