Good for fluxbox
Denu is the best menu editor for Fluxbox I've found, though it is dependent on Gentoo's Portage for application info, and therefore not terribly portable to other distros. It has a user-updatable on-line dataset of applications, which allow you to autogenerate menus (with optional icons) and categories. It has some bugs, like the fluxbox config menu doesn't currently work, and the application itself is getting a bit stale. But, again, it's the best I've found, and gave it 8 stars. Thanks for Denu!