Nvu
Now available for download is Nvu 1.0 -- a 'complete web authoring system' for Linux, Windows, and OS X. It's free, and packs quite a punch in the features department -- integrated FTP, site management, tabbed documents, spell checker, in-line preview, css editor, and numerous wizards for building tables, forms, and other elements.
Web developers comfortable with the interface of Macromedia HomeSite will definitely appreciate Nvu, for it feels a lot like it. You can browse an ftp directory on the left and edit documents on the right, all within the same interface. And when you want to see what your code looks like, you can click a preview tab to see it rendered -- still within the same window. For those who use OS X and miss HomeSite, it should feel right at home.
Now, like most open source applications, the UI could definitely use some refinement. The aqua form elements and buttons are overly large and the tabs are flat and a little kludgy looking. Nvu's UI elements feel a lot like Limewire or Firefox on the Mac, which for some people (especially those spoiled by Cocoa UI goodness) leaves something to be desired. But it's decent enough to use, making Nvu a surprisingly decent contender. And since it's free, there's nothing stopping you from checking it out.
Update: One big file format caveat I found -- Nvu forces you to edit your style sheets through its inline CSS Editor tool and refuses to open any ".css" document as plain text. Not only that, but it refuses to open other non-HTML documents as well (".xml",".js",".as"). That's really odd, and makes Nvu impossible for developers like myself to use it. Ah well, all in good time. Beta software at best.
