To paraphrase a certain jelly company...SeaMonkey is actually best described by its creators, a splinter group of the Mozilla Foundation:
The SeaMonkey project is a community effort to deliver production-quality releases of code derived from the application formerly known as "Mozilla Application Suite". Whereas the main focus of the Mozilla Foundation is on Mozilla Firefox and Mozilla Thunderbird, our group of dedicated volunteers works to ensure that you can have "everything but the kitchen sink" — and have it stable enough for corporate use.
If you have ever used Netscape Navigator or any of the non-Firefox web browsers from Mozilla, SeaMonkey is going to look very familiar. Users have a choice of web browser (Navigator), Mail/Newsgroup client, an HTML editor (Composer), Address Book, and an IRC chat client, all through a single interface. While none of these are terribly exciting in and of themselves, the interface is pretty clean and is much faster than some of the older Mozilla products.
In particular, Composer is an outstanding tool for introducing HTML to students and staff who want to post course content online. They can use the same Firefox-ish interface for browsing as well as writing web pages and the WYSIWYG editor is very easy to use. It also produces fairly clean code, at least by the standards of Word or FrontPage, so provides an easy platform for experimentation. Students will probably bump up against its limitations fairly early in a web design class, but average staff as well as students who simply need to generate content for non-computing classes will find that it is quite sufficient.
One of the most noteworthy features I discovered, though, was its installation under Ubuntu. No command lines, no sudos, no nothing. Double-click on the appropriate shell script and you're in business. Just like Windows/Mac for those of us who don't see the command line interface as Linux' strongest feature. Hopefully this is a harbinger of more user-friendly installs in Linux-land for a variety of software. For now, enjoy SeaMonkey in Windows, Mac, Linux, and even Solaris.