Yesterday I gave a talk at the Stackoverflow Devdays conference in London and as one example of the powers of YQL, I wrote a GreaseMonkey script that enhances the Stackoverflow question form to automatically add tags to the question. You can see a screencast of the script or install it yourself from GitHub. The script uses the Yahoo Term Extractor API and some lines of JS to make that work.
The demo page shows you how to do that in your own documents, all you need to do is embed the script and give it the IDs of the form fields:
There are some ways to customise the script and its behaviour, all of which are described on GitHub. Download the script from GitHub and read the docs there.