There are already a lot of useful plug-ins for downloading or uploading from online sites such as Flickr, Picasa Web, Snapfish, Webshots (disclaimer: owned by CNET Networks), and the rest, but most applications only work with one specific service.
If you want to move all of your images from one site to another (if you're one of those unhappy Flickr users, for example), and you're not looking forward to manually downloading and uploading hundreds of pictures, Migratr might be able to help. A home-brewed tool from independent developer Alexander Lucas, Migratr automatically downloads all of your photos from one site, and uploads them to the other.
It sounds great, so what's the catch? Well, Migratr only currently includes support for Flickr, Picasa Web, 23HQ, SmugMug, and Zooomr (which shouldn't even be counted because it has temporarily disabled bulk uploaders). However, downloading and uploading among the four available online photo services worked wonderfully for me. I must admit, however, that I was transferring tens of files, not thousands.
The Migratr interface and transfer process are both broken up into six steps: authenticating your download site, finishing authentication, downloading pictures locally, authenticating your upload site, finishing authentication, and uploading your pictures to the new site. It is truly that simple, and the whole process (not counting the actual uploading and downloading) takes seconds. The Migratr interface is basic but extremely simple to navigate, and the tool also transfers all of the metadata associated with any of your images.
It's worth noting that Flickr and 23HQ handle user authentication differently than SmugMug or Picasa. With the former two services, Migratr will spawn a new browser window where you must allow application access to Flickr and HQ23 via their Web sites. SmugMug and Picasa only require a valid login and password via the Migratr interface.
For me, the cool thing about Migratr is that you don't have to use it to "migrate" photos. It also works splendidly as a simple downloading and uploading tool for any of the supported photo-sharing services. To use it as a local uploading tool, simply skip to step 4, select your photo service, then pick the local directory full of images you want to upload, and you're good to go.
For quick takes on the four photo services mentioned above, I probably prefer Flickr because of the ease of use and huge community. SmugMug is definitely the most polished, with tons of options for pros, but it's also only available to paid subscribers after a 14-day trial. I find the entire Picasa experience--from desktop client to the Web galleries--to be very impressive...but I don't use it very much. 23HQ looks like another great free service, yet I've only started using it. My only comment would be that uploads to 23HQ with Migratr took considerably longer than any of the other three services I tried.