jresolver is a lightweight domain name resolver library written in pure Java. It is what RFC1034 describes as a stub resolver, i.e. it uses a single nameserver to do the hard work of resolving its queries. It operates using UDP as a transport, and has support for multithreaded querying and a robust resend and timeout mechanism.
id3j is a lightweight and easy-to-use library to create and add ID3 metadata tags to MP3 files. It only concerns itself with ID3 version 3.2.0 and only handles the most common types of information, but creating a tag and adding it to a file is much easier than with many other solutions. It also has one fairly esoteric feature that is interesting from a algorithmical standpoint, which is the ability to create CRC32 neutral ID3 tags. That means that you can add such a tag to any MP3 file and the CRC32 sum of the resulting MP3 file with the tag will be the same as the file without the tag.
rjmailer is a replacement for the SMTP part of Sun's javamail library. It sends email messages via SMTP and has the ability to connect directly to the receiving mail servers instead of using a relay server, which in many cases can result in more detailed information about the delivery status.