CraftMail
JavaMail Client [Open Source, GPL]
Grendel
Project to build a Java mail/news reader. Based at mozilla.org. [Open Source, MPL]
HotJava Views
A set of tightly integrated corporate communication tools for email, calendar management, name directory and web browsing. Supports Windows, JavaOS, and Solaris. [Commercial]
ICEMail
An email client written in Java and based on the Java Mail API. [Open Source, GPL]
Knife
Mail and news client, with support for SMTP, IMAP, POP3, Unix Mailbox, and NNTP. [Open Source, GPL]
Mailpuccino
Free, fully-featured email client written as a Java application: Supports POP3 and SMTP, and has an address book, features to view/send/save attachments, delayed posting, etc. [Open Source, GPL]
Pooka
A JavaMail based email client. Supports IMAP (including disconnected mode). Requires Java 2. [Open Source, GPL]
POP3 Mail Viewer on J2ME Devices
A fully functional program which can be used to retrieve POP3 Emails from J2ME based Cell Phones. [Freeware, source available]
PopKorn
Standalone java application that lets you check and fetch mails from a POP3 server. [Open Source]