Tomboy-Todo
Tomboy is a personal note taking system for the GNOME desktop.
I extensively use markers such as FIXME, TODO and XXX when writing so I wanted Tomboy to find and highlight those terms when I write notes, just like my favourite text editor does.
This feature not being built-in, I wrote an add-in. Mandatory screenshot:
Editing notes with FIXMEs and TODOs.