I've written many open-source libraries and gems, and have poked around with other projects, but these are perhaps the highlights.
A Heroku add-on for the Sphinx full-text search tool, which I've been running since 2011.
An open-source library for using the Sphinx full-text search tool with Ruby on Rails applications (and ActiveRecord in particular). I began this project in 2007, and it's still under active development.
An open-source library for building clean, simple and object-oriented Rack APIs with Ruby. I discuss my reasoning about it in this blog post.
An open-source library for testing Ruby on Rails Engines somewhat elegantly. I announced it on my blog quite some time ago.
An open-source library for tagging ActiveRecord objects in Ruby on Rails applications, with a focus on simplicity. As I've discussed previously, the code for this project was initially written as an example for a RailsConf workshop on writing gems that I ran in 2013.
Drumknott provides hosted, embedded search for Jekyll sites - built initially to handle searching on this site, and then turned into a service for others to use as well. I've written about it, and the code behind it is open sourced.