Trove
- Contains a Class File API to simplify the creation (and modification!) of Java class files - high-level methods are provided in order to avoid having to write bytecode directly. [Open Source, BSD-like]
- http://teatrove.sourceforge.net/trove.html

jclasslib
- A Java class file library also contains the popular Java byte code viewer. [Open source, GPL]
- http://www.ej-technologies.com/products/jclasslib/overview.html

Barat
- Compiler front-end for Java, supports static analysis of programs, parses source code and class files, builds full abstract syntax tree from source code files, enriched with name and type analysis information. [Open Source, BSD License]
- http://barat.sourceforge.net/

Package gnu.bytecode
- Contains classes to generate, read, write, and print Java bytecode (.class) files. Part of the Kawa project. [Open Source, GPL]
- http://www.gnu.org/software/kawa/api/gnu/bytecode/package-summary.html

ByteML
- A Java bytecode/XML intreroperability framework. Converts a .class file to a XML file. [Open source, GPL]
- http://byteml.sourceforge.net/

Byte Code Engineering Library
- Implementation of an API for reading, writing, and modifying class files in an object-oriented fashion. Can be used to create (dis)assemblers, (de)compilers, and obfuscators. [Open Source, LGPL]
- http://bcel.sourceforge.net/