Of the major sites, only Fictionwise and BooksOnBoard (OverDrive only) allow the Kindle PID to be used. Local libraries also use Overdrive and should accept the Kindle PID. Fictionwise does not necessarily use Overdrive, but works with Kindle PIDs anyway. See
A Beginner's Guide to Kindle Content.
Note that there is also the option of breaking the DRM with mobidedrm. This isn't as clearly legal in the US as Igor's script, but it is difficult to see who has standing (or the interest) to sue you if all you do is read the cracked ebook on the Kindle.