[dexterbt1]

The question needs expansion. What is the OP's definition of best? Why does it matter to the OP that it's open source (would your org contribute back)? Would specific family of licenses matter? And in what problem domain do the tools be used for?

I'm guessing most of HN's audience would be using some form of OSS. The coverage of the comments here is large: devops tools, programming tools, libraries, productivity, etc.

Our own org uses a lot of open source components, from desktops, office suite, dev environments, server OSes, programming platforms, libraries, databases, CMSes, heck, does Android count?

More context is better.