Great question. I went over the wall, from big law firms to open standards work, a decade ago and have loved every minute of it. (I'm the general counsel of a standards org; my definition of virtuous projects probably is broader than some.)
Lots of great project suggestions already in the comments, here. Might also want to consider the "start-up" side, hooking up with a newer or smaller project, and helping to grow it.
-- Shop around. Look for something that ignites your passion. Be aware that the threshold for declaring an organization or "project" is pretty low. Not every SourceForge page is going to be the next Apache. Invest your volunteer time in something which might have legs. The more active a community of contributors, the more likely you'd be useful.
-- Open, collaborative projects generally have accessible archives. You can read up on the issues and personalities, assess possible gigs with somee advance insight. Many also have face-to-face gatherings, in this sector, sometimes on multiple continents. They tend to be friendly and accessible. Go see the tribe live.
-- FYI, some people do make a living at it. There are a bunch of orgs where the lawyer help followed this path: started as a volunteer thing, but then evolved into full-time, self-invented, cool careers.
-- Yes, these projects often need fairly simple, non-patent-prosecutor, lawyer help. Like basic contracts, organizational (company/entity) formation, work-for-hire arrangements, basic licensing, website hosting contract review, etc.
-- There are boobytraps, though. If you act as someone's lawyer, obviously you're likely liable for them and yourself within the defined scope of your work, pay or no pay. So get the scope limits, expectations and any conflicts issues written down and crystal clear, in a few short pithy sentences, up front.
-- A word about being "general counsel": that title often is taken as a broad duty to represent the org. Be careful what you promise. Still, there are some great people having a spectacular, intellectually rewarding time being the volunteer or part-time general counsel for worthy dot.orgs like ID Commons (@DanielPerry), Linux Foundation, OpenID, FSF, IETF and other groups.