rethumb
rethumb is a service that creates image thumbnails just by accessing an URL.
It's free and host all your thumbnails.
How it works?
If you have an image at...
http://factor45.org/images/beach.jpg
...and want to create a thumbnail from it with 100px of width just access...
http://api.rethumb.com/v1/width/100/http://factor45.org/images/beach.jpg
First access will create the thumbnail and cache it for future use. The output of the HTTP request will be a thumbnail with 100px of width.