Guava

I really like the Guava libraries. One of the best things in there is ImmutableList. I like the idea of using ImmutableList on an interface. This way you don't have to worry about synchronizing the list because the list won't change on you and throw a concurrent modification exception. The builders are awesome. Unlike Apache Commons Collections it supports Generics on collections which is awesome. There's other Immutable collections and objects in this library too.

I do find a lot of other useful utility classes in Guava but it is not as complete as Apache Commons Lang and Collections. So I find myself using all three.