MagicMenu changes the look of the menus. When using this program you get a more stylish 3D-look.
ReKeyIt can change and add keyboard shortcut to the Workbench menus.
I've heard that Toolsdaemon won't work with OS3.0 but if you've got som ARexx skills you can write your own Toolsdaemon!
When it comes to adding new Workbench functionality to the already existing menus (such as sorting files by their file extension)), I don't know of any program that does this. Maybe you're thinking of Directory Opus Magellan or Scalos?
You should keep in mind that many Amiga files don't have a file extension or uses many different ones for the same kind of file (for example: .JPG and .JPEG, .doc or .guide or .readme, .IFF or .ILBM etc.) so I personally doubt the usefulness of this kind of function :-)