Personally, I do not like auto-fill tools, and toolbars for that matter. Aside from the loss of screen real estate, there's too much bloat that comes with them. Also, with the way browsers versions are increasing, auto form fill applications are sometimes not supported in newer, more modern browsers.

I've worked in Government, Law enforcement, health care, and other public and private institutions and I have yet to see a good working form autofill tool, and if I did find a good one I can grantee that someone will be calling tech support because they submitted X amount of items with the exact same data.

HTML Forms can be built many ways, and forcing someone to build it a specific way is going to limit people, thus a form should be able to be built however someone wants, hopefully following W3C standards.

That being said, the most intuitive ones are those built into an application - where the developers/BA's create the auto-fill rules based on business cases and the correct algorithms, where users can define specific fields and parameters for data in those fields. Forcing an application to be built to match a 3rd party auto fill tool, which could change at any moment, or not be supported in the future, seems risky, I hear bells.

Update:

As far as revenue concerns, or a revenue stream for such a venture, you have to have an insight on the types of users that would use this software.

A form filler needs to be more than a generic: "This is a login page, let's put a username / password in". or a contact page "This is the previous data you used for a contact me page, fill it in".

A previous system I developed was an Action Item tracking system, with build in workflow / document management. Users asked for an auto fill for these items, which on the first request seemed utterly insane (demented is the word I wanted to say, but my manager helped me keep it bottled up). How would an auto-fill utility know exactly what to fill - but as I talked to the customer they expressed the following, which is valid for all autofill tools:

When I enter in a value say "Jane Smith" for "assign this task to", it would be nice if your system would automatically put "In Progress" for the item, as I always select "In Progress" as the status for this user.

As well, this worked for other users and fields as well. There was a specific flow on how this user entered data. "Jane Smith" items were always set to a specific department, status, and if the Item Type was say "correspondence" the Estimated Time was always 8 hours.

That type of auto-fill is what we custom made for them, and they payed well for it because it saved them a lot of time, mouse movements etc. AutoFill the way it is now is annoying at best for some people. But it's the pattern of the data that matters. It has to be intuitive and learn.

Once we developed this (it was easier because it was our application, we knew what was going on), about 90% of our customers jumped on board in the first week because of the time savings, sanity savings, and they didn't need to do ANYTHING to set it up - which was key.