David Heinemeier Hansson extracted Ruby on Rails from his work on Basecamp. He released the framework as open source in July of 2004, but didn't share the commit rights until February of 2005. He's infamous for his ruthless delegation, which is often executed as a request to "Please Do Investigate". He's a partner at Basecamp, his personal page is heinemeierhansson.com, and you can follow him on Twitter @dhh.
Jeremy Kemper is a programmer at Basecamp hailing from Pasadena, California. Hot on the heels of DHH, he has been the most active contributor to Rails. He's knee deep in pretty much all aspects of the framework and one of the top batters against new, incoming tickets. You can follow him on Twitter @bitsweat
Michael Koziarski is a software consultant based in Wellington, New Zealand. After a successful stint as an enterprise Java developer, he switched to rails shortly after the first public release. He’s a contributor to The Rails Way and maintains a personal blog.
Yehuda Katz is a member of the SproutCore, Ruby on Rails and jQuery Core Teams; during the daytime, he works at Tilde. Yehuda is the co-author of the best-selling jQuery in Action, Rails 3 in Action, and is a contributor to Ruby in Practice. He blogs at yehudakatz.com and can be found on Twitter as @wycats.
José Valim is the lead-developer and co-founder of Plataformatec. He started working with Rails in late 2006 and began contributing actively after his Google Summer of Code project in 2009. He's frequently flying between Brazil and Poland, some say that it is his "eureka" time to code and bring new open-source projects to life. Check for yourself what he's up to on his company's blog.
Santiago Pastorino is one of the lead developers and co-founder of WyeWorks from Montevideo, Uruguay. He started working full-time with Ruby and Rails in the middle of 2008 after years of Java development. Since early 2010 he is trying to work as much as he can on OSS and sporadically writes on his company's blog. Follow him on Twitter at @spastorino.
By day, Aaron Patterson (tenderlove) is a mild mannered programmer for AT&T Interactive whose Ruby code is almost as good looking as him. By night, he dons his Pink Warrior suit which gives him the power to contribute to many Open Source projects such as nokogiri, Ruby, and Rails. If you have seen someone kissing Matz or DHH at a conference, it was probably him. You can find him on twitter, or read his blog.
Xavier Noria (fxn) is an everlasting student and father of the most wonderful girl, as the picture to the left clearly demonstrates QED. An independent Ruby on Rails consultant from Barcelona, Xavier has worked with Rails since 2005. He wrote Rails Contributors and has been a regular contributor for years. Honored to have been presented a Ruby Hero Award. Xavier is @fxn on Twitter.
Jon Leighton is a freelance Ruby consultant from Oxford, UK and (probably) the only core member who defiantly refuses to use a Mac! When he occasionally goes AFK, Jon enjoys dangling from small bits of rock on the sides of cliffs and mountains. You can follow @jonleighton, or read his blog.
Rafael França is a member of Plataformatec's development team. He has worked full time with Rails since 2010 and has been contributing to the framework since the beginning of 2012. As a result of being passionate with OSS, he performs a key role in maintaining his company's projects (like Simple Form) and also contributes to many other projects. Follow him on Twitter, on Github, or read Plataformatec's blog.
Andrew White is one half of Pixeltrix, a design consultancy from Coventry in the UK and has been contributing to Rails since 2007. As a passionate advocate of OSS, Andrew has contributed to many projects over the years and has been publishing his own work since the late 1990s. You can follow him on Twitter and GitHub.
Guillermo Iguaran began using Ruby in 2008 and has been working with Rails since 2009. He started contributing to Rails framework and open-source world in 2011. You can find him on conferences giving away colombian coffee to speakers. He's working in his next super-secret project and completing his Computer Sciences master's degree. You can follow him on Twitter or Github. .
Carlos Antonio is a software developer at Plataformatec, working on a wide variety of projects and maintaining Open Source libraries like Simple Form and Devise. He started working with Ruby and Rails in late 2007, and since then he has become an Open Source lover, contributing to many different projects. His motto is Hacking Beautiful Code. You can follow him on Twitter, check his Github and read Plataformatec's blog.
Rails core members who are no longer active in the day-to-day stuff have been immortalized as core alumni.