GmailerXP
GmailerXP is the be-all-end-all utility app for Google's Gmail service. The program allows users to do anything that they can do online, it also combines all the features of the other programs (GML, Gmailto, etc.) The ONLY Gmail app you'll ever need.

SharpWebMail
SharpWebMail is an ASP.NET webmail application that is written in C#. It uses a POP3 or IMAP servers as the mailstore and sends mail through a SMTP server. It is very simple to configure (only a few settings in the web.config file). You can compose HTML messages, search your inbox, read complex mime messages, have multiple address books and much more.