[Zambyte]

Contribute to projects when there is a change you would like to see made, and you have the skills to do so.

Edit: and to be clear, when I say it's a change that you want to see made, I mean in software that you actually use.

For example, I often write software that targets Scheme standards like R7RS, of which there are many compilers and interpreters for. I test my software by running it through many compilers. If my software doesn't work on one compiler, I check to see if it is an error in my understanding of the standard, or in the implementation itself. If it is with the implementation, I see if it is a simple enough change to make myself. If it is, I send a patch. If it isn't, I send the maintainer a notice (ie a GitHub issue or an email) with details on the error.

That is to say I don't go searching for things to contribute to. I contribute to things when I run into the boundries of what exists, and when I would like to push those boundries further.