I'm a BIG fan of Flex and have been developing applications with it since the Flex 2 beta. Not only is it a great front-end for Java applications but you can create some stunning interfaces for Salesforce.com.
We have a number of applications in Production using the Force.com Toolkit and Adobe BlazeDS. The Toolkit allows you to run SOQL queries, call web services and more. It's amazing the interfaces and mashups you can develop using the Force.com platform.
Salesforce.com just released an update of the Toolkit which includes the latest version of the Force.com web services API, enhancements and bug fixes. Information on the Force.com Toolkit for Adobe AIR and Flex is available here.
Today, James Ward, the RIA Cowboy (is he really a cowboy?), announced an update of the Adobe Developer Connection site that includes contains new Salesforce.com related videos, articles, demos and tutorials.
Hop over to Adobe's site and get started developing rich internet applications that will wow your users!