I have been using open source software for most of my life at this point. My contributions back to the ecosystem have always been a little lackluster, as my private development time is spent of weird pet projects that aren't of any immediate use even to myself. But I've filed some bugs and submitted a patch here and there and helped in triage and all that.
Of the code I gave back, I think the example I'm most satisfied with is my PHP deserializer for .NET. Which also only came about because I needed it for a yet unpublished weird pet project.