Mac: You can access your Kindle highlights from your Kindle itself, the apps or the web app, but none of those are particularly handy, especially if you use your Kindle for research. Knotes makes it easier to access those notes right from your Mac.
Knotes is very minimal, but that's part of the appeal. It has your highlights, notes and a way to search through them, but that's about it. You can then export those notes as a PDF or HTML if you want. Right now, Knotes only works when you physically connect your Kindle to your Mac to sync your notes, but support for the web service is supposedly on the way. You can test out Knotes for seven days before deciding whether or not to shell out the $7.20 for the app.