Notational Velocity
http://notational.net/From the description on the site:
The same area is used both for creating notes and searching. I.e., in the process of entering the title for a new note, related notes appear below, letting users file information there if they choose. Likewise, if a search reveals nothing, one need simply press return to create a note with the appropriate title.
If a note's title starts with the search term(s), that title will be "auto-completed". This selects the note and consequently displays it. Correspondingly, selecting a note places its title in the search area (De-selecting the note restores the search terms).
To create a new note, just type its title and press return. Edit the note as needed in the bottom pane.
To view or edit an existing note, type one or more words contained in its body or title. Reveal a note's content by using the up/down arrow keys to select it.
To make good use of NV, try to maintain one detail/fact/item per note. Notational Velocity's strength, note-filtering, is diminished when only a few notes contain most of the content in the database.