Google's guava project is a solid utility library providing many useful interfaces and utilities, and it is a very commonly added dependency for most projects. Unfortunately, they have a strong NIH syndrome and are somewhat half-pregnant when it comes to functional-programming. This library attempts round out some of the deficiencies that a functional programmer finds when using Guava.
In particular it provides Option and Either types similar to the Scala library as well as a Pair.
There also additional helper classes for common Function and Supplier operations.
Issues are tracked in the FAGE project on labs.atlassian.com