Photobucket is a useful way to store and use photos online, but it isn't a very nice site, as it's loud and full of garish ads. Photobucket Uploader is a Firefox add-on that lets you easily upload picture from your browser without going to Photobucket.
After installation, it's necessary to either sign up, or log in to Photobucket, and then any image you right click on will have an upload to Photobucket option. This then uploads the image automatically, and opens a bar with the options to view the image or go to the album organizer.
For people who keep blogs or have their own website, this is a much more efficient way of using your Photobucket account. While it is an improvement, it would be nice if it had an interface that allowed you to take your image URLs or embed codes out without opening Photobucket (although this would undoubtedly make them angry due to lost ad revenue). It would also benefit from being able to upload photos from your HDD automatically too.
Overall, Photobucket Uploader is a nice idea, but doesn't feel fully realized yet. If you do use Photobucket a lot, and use photos from the web, this is still a useful and functional app.