Guava - Google Core Libraries for Java
The Google Guava Libraries are a nice set of Java utility classes that will probably come in handy in any project. It provides immutable collections, string manipulation, handling primitives ,easier I/O, concurrency etc. it absolutely follows the Collections API requirements. It has support for murmur hash.
It is memory efficient. Its most interesting class is MapMaker, which is a ConcurrentMap builder, providing any combination of these features: soft or weak keys, soft or weak values, timed expiration, and on-demand computation of values. Check out their video in Youtube.