Apache java projects packaged as Eclipse projects with source included and JavaDoc URL setup.
This gives the benefits of library jar files combined with popup javadoc, source view of code, JavaDoc in browser etc in Eclipse. All projects export their jar files and is very efficient with e.g. MyEclipse and ant4eclipse.
This works very well with Team Project Sets which allows any developer to download the needed projects directly and transparently with CVS without having to download, extract jar and add library jar. It also allows for projects to avoid having compiled code in their own source code repositories.
Projects include version numbers in their names, but does not require other projects allowing the developer to choose the exact version needed.