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.

https://images.weserv.nl/

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