Please consider AM-Notebook. I rely heavily in work on this program and recommend it, because I'm so content with it (and I used many tree outliner).
Latest Version: http://www.aignes.com/beta.htm
It supports RTF-Notes (TRichEdit), tables (with Excel like formulas) and now also diagrams.
It never crashes (Use it everday since half a year.).
Completely Unicode, small, portable, clean interface, good export
You can have multiple tabs, but not like most tree outliner with a tab for every tree, but a tab for every note in one tree (much better IMHO).
- unlimeted undo
- documents stored as separate files on harddisk, no database that can get corrupt
- automatically save sdocuments and saves last three versions (can be more if you want)
- daily autobackup of all data
Fast open/find notes: press Ctrl+F5 start typing the name of the note, press Enter voila.
Share notes collection with other users over network.
No Candybar-Look: rather bare bone but clear and functional (I like that).