Excellent programmer's editor
I have used Epsilon for many years now (about 12 maybe?) and can't say enough good things about it. Even though it is not truly open-source most of the editor is written in its own extension language called "eel". Eel is a c-like extension language and I have found it very easy to modify the editor to my liking. Although I haven't found any reason too since about version 6 or so. Yes, it is that good. It is constantly gaining new features. You can even download a trial version from their website.
No I don't work for them or have any financial interest in them. I just really like this editor that much. Currently I use Epsilon with Linux (command line and gui) and Windows. I used to use it in OS/2 also.
An open, cross-platform journaling program.
A scientific plotting package.