BeanShell is a small, free, embeddable, Java source interpreter with object scripting language features, written in Java. BeanShell executes standard Java statements and expressions, in addition to obvious scripting commands and syntax. BeanShell supports scripted objects as simple method closures like those in Perl and JavaScript(tm).
You can use BeanShell interactively for Java experimentation and debugging or as a simple scripting engine for your applications. [...] you can call your Java applications and objects from BeanShell; working with Java objects and APIs dynamically. [...] you can freely pass references to "real live" objects into scripts and return them as results.

Very, very useful (for projects where Perl is not an option and you have to use Java). It gives you great rapid prototyping capabilities, speeding up your development (especially the testing and debugging part).

BeanShell is also used internally by the JEdit text editor to write macros.