kmMail is a PHP-based mail program, based on the mail functionality of Keftamail and is being developed by current Kefta employees. It is meant to be a framework for putting a web-based mail program on your site, but is still robust enough to drop into your current site with little to no modification. The goals of kmMail are MIME compliance, speed, low overhead, and an enjoyable user interface.
GSendMail is a simple, standalone mail composition tool using the GTK+ toolkit. It allows you to fill in From, To, CC, and Subject fields, and compose a text/plain message in a fixed-width text area with word-wrapping. It does some sanity-checking on typed addresses, but beyond that it is frill-free (no HTML, no attachments, no address book). This program makes it possible to click on mailto: links in Mozilla/Netscape installations without the mail component.