I had a personal experience with my project this week that I think is really cool and thought it was worth sharing.
I've been working on the role-playing game Hero of Allacrost since 2004 in my spare time. I've talked about it on this site many times in the past so some people here might remember it. Recent years have been rough for me and I haven't always been able to work on it. I recently came back from a 15 month break where I completely detached myself from the project. Sadly, no progress was made by anyone else during that period of time and the project has been effectively dead, so I gathered the wreckage and get it off the ground once again.
Then I learned that someone who was disappointed with the lack of progress decided to fork my project into their own, Valyria Tear, with a different story, new content, and a great number of new features that he added that were on our development task list. He's really made some awesome improvements to the game, which I am now able to backport and add into my own project. I've even decided to help him out with his project for the time being while I work on organizing my own, as working on his project is just as good for my project as working on my own directly, and he's in a better position to make some real progress right now.
I'm really happy about all this, and I feel like everyone wins. He gets a great starting point for his game with years of work already invested, I get a free "upgrade" to my project despite it being dormant for over a year, and the players get better, faster releases as a result. This is (just one reason) why I love open source software development. It has always been a personal goal for me with this project for others to be able to use our code in their own projects, and after 8 years it has finally happened.