JDBM is a transactional persistence engine for Java. It aims to be for Java what GDBM is for Perl, Python, C, etcetera: a simple persistence engine that is lightweight and fast.
Note that JDBM's development has moved on to github - https://github.com/jankotek/JDBM3 is now the new home.
Most of all I like the fact that all updates are made in a transactionally secure manner.
Great software, thank you.
Perfectly fills the gdbm niche in Java
Easy API, worked first time out-of-the-zip and great support/advice on the forums too.