Jason Fried blogging about the Fizzy launch, which is free as open source or $20/month hosted:

If you’d prefer not to pay us, or you want to customize Fizzy for your own use, you can run it yourself for free forever. Have a great idea? Submit a PR to contribute to the code base and improve the product for everyone.

37signals is very opinionated, so I wonder if too many PRs will clash with how they usually grow a product.