I've been using S3Fox to access AWS S3 for quite a while now but recently I've had trouble with it. When I launch it from Tools | S3 Organizer, it fails with an "error connecting to server" message. Launching it in "dropbox mode" from the status bar works fine, though.
I found this thread suggesting a workaround. Apparently you have to log out of your Google account before launching S3Fox. I don't know why this would cause problems, but it works for me.
I'm using Firefox 3.6.2 and the most recent version of S3Fox on Windows 98