Mainstream 

Jython

Python scripting engine

JRuby

Ruby scripting engine

Tcl/Java

Tcl scripting engine

Rhino

Javascript scripting engine

Groovy

Groovy is an agile dynamic language for the Java 2 Platform that has many of the features that people like so much in languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax.

Other engines 

BeanShell

BeanShell is a small, free, embeddable Java source interpreter with object scripting language features, written in Java. BeanShell dynamically executes standard Java syntax and extends it with common scripting conveniences such as loose types, commands, and method closures like those in Perl and JavaScript.

Jelly

Jelly is a tool for turning XML into executable code. So Jelly is a Java and XML based scripting and processing engine. Jelly can be used as a more flexible and powerful front end to Ant such as in the Maven project, as a testing framework such as JellyUnit, in an intergration or workflow system such as werkflow or as a page templating system inside engines like Cocoon.

JScheme

JScheme is a dialect of Scheme with a very simple interface to Java, called the Javadot notation . This notation provides a transparent access to all Java classes, constructors, methods, and fields on the classpath.

LuaJava

Lua scripting engine.

Pnuts

Pnuts is a script language for Java environment. It enables interaction with Java environment, simple Web scripting, customization for Java programs, and so on.

Hecl

The Hecl Programming Language is a high-level scripting language implemented in Java. It is intended to be small, extensible, extremely flexible, and easy to learn and use.

Yoix

The Yoix scripting language is a general-purpose programming language that uses syntax and functions familiar to users of C and Java. It is not an object oriented language, but makes use of over 150 object types that provide access to most of the standard Java classes.

Judo

Judo is a practical, functional scripting language. It is designed to cover the use cases of not only algorithmic/object-oriented/multi-threaded programming and Java scripting but also a number of major application domain tasks, such as scripting for JDBC, WSDL, ActiveX, OS, multiple file/data formats, etc. Despite its rich functionality, the base language is extremely simple, and domain support syntax is totally intuitive to domain experts, so that even though you have never programmed in Judo, you would have little trouble figuring out what the code does.

Sleep

Sleep is heavily inspired by Perl with bits of Objective-C thrown in. The Sleep Java API allows the language to be extended with new constructs, operators, functions, and variable containers.

DynamicJava

DynamicJava is a Java source interpreter.

Related frameworks 

Bean Scripting Framework

Bean Scripting Framework (BSF) is a set of Java classes which provides scripting language support within Java applications, and access to Java objects and methods from scripting languages.

BSF supports several scripting languages currently:

* Javascript (using Rhino ECMAScript, from the Mozilla project)
* Python (using either Jython or JPython)
* Tcl (using Jacl)
* NetRexx (an extension of the IBM REXX scripting language in Java)
* XSLT Stylesheets (as a component of Apache XML project's Xalan and Xerces)

In addition, the following languages are supported with their own BSF engines:

* Java (using BeanShell, from the BeanShell project)
* JRuby
* JudoScript
* Groovy
* ObjectScript

EclipseShell - Scripting for Eclipse

EclipseShell allows you to use

* JRuby
* Beanshell
* JavaScript (Rhino) to script your Java classes.

Eclipse shell allows you to do this by

* providing an interactive Editor with AutoComplete and other amenities, so you don't have to mess with ugly readline interpreters.
* automatically setting up the CLASSPATH to your classes and the libs, taken from your Workspace Project settings.
* making it easy to store and reuse the history of your interactive sessions

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Books 

Jython for Java Programmers (Landmark)

by Robert Bill

Amazon Price: $34.99 (as of 10/11/2008)

Practical Programming in Tcl and Tk (4th Edition)

by Brent Welch, Ken Jones

Amazon Price: $49.10 (as of 10/11/2008)

Programming Ruby: The Pragmatic Programmers' Guide, Second Edition

by Dave Thomas, Chad Fowler, Andy Hunt

Amazon Price: $29.67 (as of 10/11/2008)

JavaScript Bible, Fifth Edition

by Danny Goodman, Michael Morrison

Amazon Price: $32.99 (as of 10/11/2008)

The Little Schemer - 4th Edition

by Daniel P. Friedman, Matthias Felleisen

Amazon Price: $25.20 (as of 10/11/2008)