What are some FOSS apps you've found that made you go "wow, I cant believe this is free" and/or "I can't believe I lived without this?

Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.

MilkyTracker is an open source, multi-platform music application for creating .MOD and .