Treepad [1] - very nice, but not open-source. Freeware versions for both Windows and Linux. Shareware versions for Windows only, but Linux shareware version is in development.
Woody [2] - written in python, curses based (console mode only)
LEO [3] - outline based literate program editer. Cross-platform, written in python/Tk. This is a two-pane outliner
Java Outline Editor [4] Nice-looking cross-platform Java outliner. Supports editing more than one outline at a time.
Hierarchical Notebook - [5] Single-pane console outliner based on a curses interface.
TVO: The Vim Outliner - [6] Outliner based on the vim text editor.
KeyNote - [7] Open source, double-pane outliner with a long list of features. Windows only. Compatible with freeware Treepad files.
http://www.outliners.com. There is a nice discussion board about outliners here. Also, if you have a Macintosh, the More Outliner is availble for download.
http://www.atpm.com/9.10/atpo.shtml - Incredibly comprehensive set of 3 (as of 12/6/03) articles surveying every conceivable outliner feature and outliner applications available for the Mac.
Personal wikis
The wiki software that runs this site can be run both in web server mode and as a local mode GUI app. See http://www.equi4.com/wikit.