There are a couple of options, depending on preference.
NVU -- probably a good choice for clients doing simple edits.
Otherwise any code editor. One unusual choice you may favor is
Arachnophilia, which just got an update. Why? You can completely alter the commands, toolbar options, folders, and interface. This would allow you to strip out everything but what the client might need. Using the built in macro language, you could create user input popups which could further cut down any direct code editing.
Finally, the third option everyone else is doing is a content management system. Most developers pretty much use it as a layout editor and not a CMS anyway. The advantage is the disadvantage -- online editing. Ideally you want an editorial process step for promoting an article to insure checking before something goes live.