There's also thumbor (https://github.com/thumbor/thumbor). It's a very mature implementation of this type of server and has been very battle-tested (https://github.com/thumbor/thumbor/wiki/Who%27s-using-it).

At globo.com we have near a billion images (we are a big portal). Can you imagine pre-generating that many images every time a new format gets added?

We serve everything with thumbor with a Varnish cache in front of it and we're very happy with it. It has enabled our designers to work with any image size they can think of.

If you guys need more info, please check thumbor's docs: https://github.com/thumbor/thumbor/wiki/