One way to get recruited by a company is to contribute to their Open Source code-base before you even apply.

On this page you'll find a bunch of companies with some information about their Open Source projects.

In general you could easily find projects for most companies.

Open Source Companies

I am not really sure what Open Source Companies are.

Are these companies that have no propriatery code at all? I dobut there are many companies without any propriatery code, but I'd love to know about them.

Companies that build some Open Source project that is the core part of their business?

Companies with a significant code-base they have released under some Open Source license?

Companies that contribute to Open Source?

Companies where the CEO know how to spell Open Source?

Companies

Twitter has a bunch of Open Source projects in GitHub. As I see many of the projects use Scala, Rust, but there are a few in Python and in Golang as well. I also saw Ruby, JavaScript, Objective-C and a few more language.

In my mind Adobe was this super-proprietary company, so I was surprised to see it as one of the companies with the largest number of Open Source projects. As of today they have almost 500 repositories on GitHub. Though I am not sure how important these projects are. I see a lot of projects in JavaScript, and some in other languages such as Java, TypeScript, Objective-C, Python, Dart.

GitLab source code

Automattic the creator of Wordpress and several other Open Source projects. Automattic on GitHub, the source code of Wordpress.

MongoDB GitHub.

Black Duck Software GitHub.

DuckDuckGo the privacy oriented search engine has a separate site for Duck Duck hacking and 89 repositories on GitHub. Among the languages I can see Perl, Python, JavaScript, and Swift, just to name a few.

Grantstreet Group deals with Tax collection, auction, and e-payment solutions. Not something I'd associate with Open Source, but they too have a GitHub account with several repositories in it. The majority of the code is written in Perl, but you can also find Go, JavaScript, Python, and even CoffeeScript.

Booking.com the hotel-reservation site also has a few projects on GitHub. The languages are Go, Perl, Java, and C#.

MaxMind.com has some projects on GitHub. The languages are Node.js, Python, Ruby, Java, Go, PHP, .NET, C and Perl.