Sidenote — Note-taking program for MacOSX
Sidenote is a small app that is normally hidden at the edge of the screen. When the cursor is moved to the appropriate edge it slides out with a simple rich-text-editor, useful for quickly taking notes and such. When you move the cursor away it slides back. It automatically saves and supports multiple note-files.
There are similar web-based services, but Google Notes, for example, isn’t much good without it’s extension, and that is only on Firefox and IE to my knowledge.