Joind.in is one of the featured projects and I'm one of the maintainers, so I'll be at the hackathon and I'm hoping that we'll get quite a few things done during the evening. Joind.in is an ideal project for events like this since it's easy to get started with it, and we have a development platform virtual machine (that we'll have already downloaded onto USB sticks so no conference wifi delay) so you can be up and running in no time. We also have a specific label on our bug tracker for items that we think are manageable for people who don't already know the system, so chances are that if you want to, you'll be able to contribute to an open source project with something finished by the end of the night.
Other important things to know about joind.in: we have a code of conduct, and we welcome everyone, regardless of background, development ability, or any other variables. At PHPNW there will be a mix of experienced contributors, people who hack with us once a year at PHPNW, and total beginners, so wherever you are on that spectrum, please know that you are welcome to join us! We had a lot of fun last year.
If you're keen for a quick start on the night, then go to our about page, get the development platform set up, and take a look around the issue lists - see you on October 2nd!