With the new Automator in Mac OS X 10.4 it's easy to add contextual menu items to the Finder. Just edit your workflow in Automator, Select "File->Save As Plug-in...", enter a name for the plug-in, and specify that the plug-in is for Finder. (Apple has posted some examples of how to create plug-ins.)
It's easy to add a workflow menu item. How do you remove one?
Intuition says that your personal workflows might be represented as files somewhere in your home directory. Finder's new Smart Folders should tell whether that's really the case.
Create a dummy workflow named "DeleteMe" and save it as a Finder Plug-in. Then switch to the finder and select "File->New Smart Folder". In the little brushed-metal "toolbar" which appears, select the "Home" folder. In the criteria list add the following criteria:
"Kind", "Any", "+"
"Last Modified", "Today"
Somewhere in the list of search results a Folder named "Finder" should appear. Hitting the info icon on that item doesn't tell much, but the "More Info..." button reveals that it's located in /Users/YourUsername/Library/Workflows/Applications/Finder. And lo, opening the folder shows an Automator Workflow named "DeleteMe".
Drag the workflow to the trash, right click anywhere in a Finder window, and select the "Automator" cascading menu item. The "DeleteMe" menu item should be gone.
The Short FormWorkflow menu items live in your ~/Library/Workflows/Applications/ folders. They can be removed from your context menus by dragging them to the trash.
There's probably a better way to discover all of this...