Image cache and resizing. You can use the hosted service directly to resize & crop images on the fly, or run a version of the open source code on your own server.
Example:
<!-- images.weserv.nl/lichtenstein.jpg -->
<img src="//images.weserv.nl/?url=images.weserv.nl/lichtenstein.jpg&w=300&h=300">
There is a Quick Reference page with all the arguments: https://images.weserv.nl/docs/quick-reference.html
And the Size page is likely to be the most widely used: https://images.weserv.nl/docs/size.html