Granite Data Services (GraniteDS) is a comprehensive development and integration solution for building Flex / JavaEE RIA applications. The entire framework is open-source and released under the LGPL v2 license.
Starting with the new 2.2.0 GA release of the framework, you may use a full-featured ActionScript3 implementation of the Bean Validation (JSR-303) specification. Validation of Flex beans can now be controlled by constraint annotations placed on their properties, just like Java beans. Furthermore, integration with GraniteDS code generation tools allows on-the-fly replication of your Java constraint annotations into your ActionScript3 model. See more in the online documentation here.
Additionally, the 2.2 release comes with a reliable implementation of big numbers (BigDecimal/BigInteger) in ActionScript3: you may compute with arbitrary precision and precise rounding mode control on the Flex side, and serialization between Java and Flex is accurate. This feature may be a big asset for bank or scientific applications. See more in the online documentation here.