Open source development has never been more prevalent and successful than it is today. The biggest players regularly publish the latest and greatest technology with permissive licenses. Some companies even do their whole development in public — inviting anyone to fork, download and send pull requests.
Google has always been a strong advocate of open source and even funded a lot of non-Google open source projects through their Summer of Code program. Facebook publishes not just software, but even the specs to their servers.
Microsoft recently joined the party and open-sourced key parts of their technology stack and the company even develops core pieces openly on Gitgub.
The big question is whether or not the agile development style mixes well with open source. On the face of it, they are polar opposites. Agile evolved from tightly knit co-located teams. Open source is all about strangers who have never met collaborating across the globe.
All the same, there are many similarities and shared principles between the two methodologies. Both paradigms put a great deal of emphasis on testing and automation. Both favor small and quick iterations. Obviously, co-located Agile teams can publish the results of their work as open source. The more interesting case is when an agile development team, that is not co-located, can still successfully take advantage of open source methodologies such as rigorous source control policies, pull requests and continuous integration tools.