The jakarta commons project is a great example of reusability. It’s a collection of utilities that can be used independently. The only problem is a lack of documentation. There are some books available. I’ll look into these as soon as possible (just ordered a copy of the jakarta commons cookbook, ISBN:059600706X).

Some I used so far are:
commons.lang.StringUtils
commons.lang.builder.*
commons.collections.CollectionUtils
commons.collections.Transformer

…

For anyone who wants to get involved:
http://jakarta.apache.org/commons/volunteering.html

resources:
http://www.onjava.com/pub/a/onjava/2003/06/25/commons.html
http://jakarta.apache.org/commons/
http://www.javaworld.com/javaworld/jw-07-2005/jw-0725-chain.html
http://www.javaworld.com/javaworld/jw-10-2002/jw-1025-opensourceprofile.html