Paperclip is an interesting idea for an application. Basically, it looks like Stickies but the notes are actually "clipped" to specific documents in other applications. For example, you can create a Paperclip note and attach it to a Pages document, then whenever you open that document the relevant Paperclip note will appear pinned to the side of the window. The notes themselves can contain rich text, links, images, etc.
Paperclip does this by running as a menubar application with Universal Access active. In my brief testing I noticed that it will not work correctly with an unsaved document. For instance, if you create a new, untitled Pages document and try to attach a note, it will not work. If you save the document, however, it seems to work fine. I think this is a neat idea, and it could definitely be useful. The Paperclip notes are basically a form of meta-data, so its unfortunate that it doesn't integrate with Spotlight. Nonetheless, I like the concept and it will be interesting to see where it goes.
Paperclip is $9.95 (introductory price) and Leopard only. A demo is available for download from Concentrate Software.