Why you should upgrade to V3

V2 is deprecated; upgrade to V3

Over 300 bug fixes and performance enhancements that simply aren't possible with the V2 architecture.
Simpler, yet far more capable API.
Hundreds of new features.
Free updates. V2.8 support ended on June 15, 2011. V2.X was supported for 3 years, folks.
48+ plugins are available for everything from PSD parsing to SQL or S3 backends.
Simple, intuitive configuration. No more polluting <appSettings>. Go from 2 complex pages to 6 easy lines.

V2 has no free version. V3 has a free version, but it does not include disk caching, octree quantization, dithering, or animated gif support. Those features are included in the Performance Edition.
V3 has a very slim 'core'. Extra functionality is added with plugins. Plugins are easy to write, and can affect any aspect of the pipeline.
V3 is easy to extend without modifying the source code. V2 was designed for source code modification.
V3 is configured within the <resizer /> element, not the <appSettings /> element.
V3 has good default settings. Configuration is optional.
In addition to the 30 new paid plugins, 14 new plugins have been included in the free core download of V3.
V3 makes URL rewriting easy.
V3 has a self-diagnostics page. Discover configuration and coding errors by visiting /resizer.debug.
The V3 core is under MIT-like license, which is clearer and offers much more freedom than the 'V2' license.
Unlike V2, V3 has a clear (and permissive!) license for enterprise use.
Version 3 offers disk caching and gif quantization as plugins, not in the core. This allows other kinds of caches to be installed, and also allows the core to be completely free and open-source while still providing a profit model for maintaining and improving the resizer.
Fallback images. Specify fallback images in case a file is missing.

The V3 (and V4) "Performance Edition" includes everything users of V2 expect, and more.