For Java, all of Apache's stuff is good - particularly Jakarta. http://jakarta.apache.org/ This covers a wide range of topics and is designed to have a public API. http://code.google.com/p/google-collections/ and http://code.google.com/p/guava-libraries/ and http://code.google.com/p/google-guice/ are interesting in that they come out of Google, beyond their own individual merits.