I really like Bluefish. It's comparable to FirstPage (windows freeware program), with:
A tree-view listing of files/directories, syntax highlighting for HTML, PHP, PERL, java, python, and others
References for HTML and PHP
Toolbars for quick insertion of lists, forms, tables, images, anchors, etc.
It also uses the tidy library (not included, but simple to compile) to clean up your HTML with the push of a button (and options to let you review it's edits).
The only thing I can think of that's lacking from Bluefish is an FTP function to let you easily update your site. And in the interests of full disclosure, I should mention that it's not WYSIWYG, but I never had much use for the drag'n'drop method of creating webpages. If that's more your cup of tea, you might take a look at Mozilla's editor. It's surprisingly full-featured, but I can't really give much of an evaluation of it as I haven't used it since I found Bluefish.