Adobe released a number of open-source ActionScript 3.0 libraries. Some of them are very useful, some are more application specific. But they are all good for learning the new coding style.

corelib includes a great MD5 encryption class that could pretty easily be ported back to AS 2.0. There are also a nice serialization class in there.

FlexUnit is a unit testing framework (similar to JUnit) for ActionScript 3.0 (not just for Flex). If you're not familiar with Extreme Programming and Unit Testing, check out AS2Unit for a great tool to apply unit testing to your ActionScript 2.0 projects.

The RSS and Atom libraries read various types of RSS feeds (and automatically manage parsing the XML based on the type of feed). Cool stuff!

Available Libraries:

corelib
FlexUnit
Flickr
Mappr
RSS and Atom libraries
Odeo
YouTube