For example, you may be writing a document and research something on the web. Hook would allow you to link a URL that was useful to your document. Then again you could link to an outline, a presentation, or an email.
This screencast (6m58s):
shows Hook in action. I think it’s quite an interesting tool.
In practice Hook is based on using AppleScripts to drive various applications to get the name of the in-focus document, and to re-open different documents.
I think Hook is sufficiently interesting and sufficiently cheap that I’ve just bought a copy to play with.