Ask YC: Underrated open source software?

[RexEast]

I'm looking for examples of great open source software that is underrated, owing to a lack of proper advocacy, strategy, or "marketing". For example, I am currently working on a project in which all the developers are solely interested in working on the software; as a result, the website hasn't been updated in 7 years, and up until recently the Windows and Mac download links were buried somewhere in a wiki page. I recently put in a few days' work to redesign the site, and I think the impact of this redesign will be pretty big.

It has been very satisfying to work on this project, and I'm looking to find others that could use some help. Can you guys think of other projects like this, especially those that have websites that scare off or confuse new users?