Windows: Amazon has released a Windows application, Kindle for PC, that tells you everything in its name. It syncs with other Kindle devices, offers color pictures, font size control, and other controls for reading on your computer.
Amazon seems to be pitching Kindle for PC as a supplement to, not a substitute for, a standard Kindle experience. Kindle for PC doesn't offer blogs, newspaper, or magazine subscriptions, and the interface doesn't offer much over the Kindle device it sprung from. That said, those without iPhone/ipod Touch or Kindle devices might be eager to try out the instant-on e-book experience, along with the joys of reading through free sample chapters for any and every book.
Kindle for PC is a free download for Windows XP, Vista, and 7 systems only. A Mac version is "coming soon," according to Amazon.