Nice is an object-functional language which extends Java with:
- parametric polymorphism
- multiple dispatch
- abstract interfaces, which can be added to a class after the fact
- first class functions
- a much stronger type system
This is a language which is very accessible to Java programmers and may help to show you what you've been missing by not investigating functional languages.
Also be sure to check out the Nice wiki site
You can download the Nice compiler from the install page. If you're a regular user and you want to try the bleeding-edge CVS version, you can get that here.