Fire Uploader is a Firefox extension that lets you upload files to the most popular image/video/document sharing sites from a single interface. Most sites only let you upload a file at a time, but in Fire Uploader you can just drag and drop all the files from a folder.
The extension supports YouTube, Flickr, Picasa Web Albums, Webshots, Box.net, Omnidrive, Facebook, but the author plans to add more sites.
For Picasa Web Albums, you can see the existing albums, create new albums, upload and download photos and delete some of them. While the option to upload photos is also available in Picasa, this extension lets you manage the online photos in an interface similar to a file manager or an FTP client. Note that all the albums created from the extension are public, but you can change that from Picasa Web Albums.
You can also upload multiple videos to YouTube, backup your Flickr photos, download only some of the files from Picasa Web album and manage multiple accounts for a single site. The interface is pretty rough and there aren't too many options you can configure, but if there are so many powerful download managers, why not have an universal uploader for all the sites that store files in the "clouds"? The meaningless power of a single channel to the "cloud of half-promises and unseen miracles"*.
* From "Det sjunde inseglet" (available for free at Google Video)
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