imgscalr is a small, fast Java library using Java's best-practices for image manipulation and providing simple static-method implementations of a handful of core image manipulations (e.g. resize, pad, rotate, crop, etc.)
imgscalr was written and is maintained by Riyad Kalla. It is released under the Apache 2 open source license and is used in a number of commercial applications.
imgscalr was originally written and released in response to seeing the same questions about "good looking thumbnails in Java" and seeing many different ways to accomplish the same thing; some solutions being better than others.
imgscalr took all the best-practices (lowest memory overhead and fastest performance) and rolled them into a single, simple class that exposes a number of static methods that any caller can use to manipulate images.
The popularity of the library grew and the addition of a few other utilities continue to be added as long as they meet the criteria of "being simple to understand and use".