SeaMonkey is a web browser that uses the same rendering engine as Firefox, sharing some of its features as well, like tabbed browsing and a pop-up blocker.
The software includes an e-mail client and Thunderbird-based newsgroups compatible with .Mac and Unix Mailspool (Movemail) accounts. It also supports multiple email accounts and includes a junk mail filter to avoid spam. On top of all that, SeaMonkey offers an IRC client: Chatzilla.
SeaMonkey also offers other features that might be very useful for web developers, like a HTML editor (Composer), a Javascript debugger, and a DOM inspector.
