Amazon released Kindle software for Windows. It is only in Beta right now but it’s free.
# Get the best reading experience available on your PC. No Kindle required
# Access your Kindle books even if you don’t have your Kindle with you
# Automatically synchronizes your last page read and annotations between devices with Whispersync
# Create bookmarks and view the annotations you created on your Kindle
It’s cool that it will start you from the last page you were on. The Mac version is coming soon.