I have a channel that will require lots of editing (adding and selecting) of many categories and want to make this as simple a process as possible for the client. I also need to require that only one category can be selected at a time (unlike the multiple on the Categories tab).
Is there a decent fieldtype add-on that allows for managing categories, not just selecting them?
You can try my Category Field addon which should do exactly what your looking for:
This field type allows you organize your category groups as you would any other custom field type anywhere on your publish entries form. Additionally a filtered input is provided for long category lists.
This fieldtype is often overlooked, but I find for single-category entries, Lonely Cat by Wouter Vervloet to be amazingly simple and effective.
http://devot-ee.com/add-ons/lonely-cat
The advantage of Lonely Cat is in its ability to be instantly grasped by the user, and in the fact that it selects only one category at a time, though it can be configured to also select all parents of that category, as well.
I would suggest you use Playa for your categories. That will make it easy for your client to edit/add or delete categories.
The Category Field add-on mentioned by Nuno looks like it may cover it.
The Solspace Primary Category add-on provides another option for managing category selection, though likely not as in-depth as you are looking for.
http://devot-ee.com/add-ons/primary-category
Sometimes I also use Tags as an alternative for categories.
None of the add-ons really did the trick so I ended up just dragging the categories field onto the Publish page from the Toolbar.
