There is a large variety of what is commonly called 'Content Management Systems' out there. They range from simply web-based FTP clients with file editing ability (ran as a perl script or PHP app on the server), all the way to complex systems that use templates for formatting, and do all the basic tags, p, br, b, i, etc for you.
Search around for WYSIWYG web based scripts, web based ftp programs, web based text editors, and you'll find literally hundreds of various options, most of which are free. Usually they require someone of at least moderate technical skill to install (some are easier than others, of course), but then can usually be operated by anyone who has at least a rudimentary understanding of how to use the web.
Peace,
sam