Amazon just released an update for the Kindle PC desktop software. New features include adding and editing notes and highlights, adjusting brightness, full-screen view, and changing the background color to white, black, or sepia.
The changes give Kindle for PC pretty much the exact same functionality as the Kindle iPad app, which is reviewed on this page.
Overall, the Kindle for PC app is a decent free ebook reader, with access to the most comprehensive library of ebooks on the planet, and a lot of free ebook titles as well (here's a free ebook by Robin Hobb). Amazon is definitely stepping up their game in recent months to keep up with all the new competition.