Java

CraftMail
JavaMail Client [Open Source, GPL]
http://www2s.biglobe.ne.jp/~dat/java/applications/CraftMail/index_en.html

Grendel
Project to build a Java mail/news reader. Based at mozilla.org. [Open Source, MPL]
http://wiki.mozilla.org/Grendel

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]
http://java.sun.com/products/hotjavaviews/index.html

ICEMail
An email client written in Java and based on the Java Mail API. [Open Source, GPL]
http://www.trustice.com/java/icemail/

JVMail
Freeware Java SMTP command-line client.
http://www.quacknews.org/index.php?JVMail_Project

Knife
Mail and news client, with support for SMTP, IMAP, POP3, Unix Mailbox, and NNTP. [Open Source, GPL]
http://bluezoo.org/knife/

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]
http://www.kingkongs.org/mailpuccino/

Polarbar Mailer
A client is presented with features, screen shots, Download, change logs, installation help and FAQs. [Freeware]
http://www.polarbar.net/

Pooka
A JavaMail based email client. Supports IMAP (including disconnected mode). Requires Java 2. [Open Source, GPL]
http://www.suberic.net/pooka/

PopKorn
Standalone java application that lets you check and fetch mails from a POP3 server. [Open Source]
http://members.tripod.com/~SandsOfDee/PopKorn/PopKorn.html