Ubersense-hackfest is a project mainly written in ..., it's free.
The result of 24 hours of hacking @ Twitter Annotations Hackfest
In order to run the Flex application, download it and place it on your local file system.
The directory that holds the Flex application needs to be a "trusted site". Use the Security Manager to mark that directory as trusted: http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager04.html
Enjoy!