How to find volunteer or similar open source type work?

I am interested in finding some projects to work on a few hours here
and there to keep up my skills and perhaps add to my resume. I do alot
of backend work and I do not have as big of a visible online portfolio
as some people because I have done alot of things involving internal
web portals for various companies.

My impression from looking around a bit is people find some gem or
app on github, they download the code and play with it, modify it and
push it back up to see if the repository owner might want to use their
changes. Are there other approaches to this ?

I might be interested in finding people who have an actual need for
some work on their web site, some text processing, database, whatever
it may be. Otherwise I have to try to figure out what some gem does
and then figure out why I would want to change it and ask myself would
other people want this change without interacting with anyone who has
a specific need that they communicate to me.