I'm a fan of ColdFusion, which is built on top of Java, but with a more RAD/Agile focus. There's the reference implementation from Adobe, but there are also three open source implementations of the CFML language which are available.
ColdFusion is particularly good at being a "Glue" technology, providing access to Flash/ActionScript services, Java Integration, .Net integration, web services, and SQL database integration using any JDBC datasource.
