Scintilla is a project, led by Neil Hodgson, to produce an open source text editor component for GTK+ (UNIX) and Windows.
The project's flagship is SciTE, a text editor application that uses this component.
Scintilla and SciTE are particularly interesting when editing source code. It looks nice and provides visual aid for coding - folding is supported, as is code completion and styling (and coloring) of code.
I haven't used SciTE much, but it appears to be pretty good - definitely beats nedit on UNIX (hey, nedit has those ugly and unusable Motif file requesters, GTK+ has better ones!), Notepad (not that it would take much =) and probably even PFE and several other editors in Windows - I'm not sure if it's good enough to beat XEmacs, though...
Home page: http://www.scintilla.org/
Needless to say, SciTE is also available as a Debian package.