Sidenote is a clever little note-taking app that comes in the form of a drawer which slides out from the edge of your screen. Giles Turnbull at O’Reilly’s macdevcenter likes it 
and so does C.K. Sample and Rui Carmo.
An updated version (1.7.1) released today is a universal binary and also saves its notes repository in a more traditional place (~/Application Support/Sidenote), making backups easier.
The app’s window slides out from the side of the screen when you move your mouse over it or activate it by a user-definable hot-key.
It supports images, PDF files and rich text format as well as plain text.
You can access previous notes from a drop-down menu at the top of the interface. Preferences allow you to set the drawer’s height, the popup/popout delay, drawer’s opacity and whether to display the screen edge border. Further hot-keys can be assigned for clipping text, browsing notes, creating, saving or deleting a note.
Its auto-saving feature reduces fiddle and saves you from remembering to do it yourself. Getting the notes out again is just as easy; it has full print support and allows you export the notes to RTF.
UPDATE: A further update (1.7.2) will now run on 10.3 as well as 10.4. The current version only works in Tiger. The developer hopes to restore compatibilty with 10.3 soon.
Sidenote is freeware (donations not refused) and is available from the developer’s web site .
While I love little apps like this or Voodoo Pad in principle, I need a cross-platform solution. A hosted wiki is what I’ve ended up with as a way of being able to save snippets of (non-structured) information includeding formatted text, plain text and/or images. Stikipad is the one I use. In addition to a very nice UI, there’s a simple built-in way to back it up by downloading linked HTML files of the entire wiki. PBWiki has a similar mechanism with a single zipfile incorporating the whole wiki, downloadable with a single click.