I had earlier written about a ubiquity command that helped in quickly bookmarking pages. The command posted the bookmark to delicious with the selected text as notes. The command does not support tags yet.
Adding tags manually was always a turn off; that is the reason I was searching for online tag generators for web pages. I wanted some service to which we could pass a url, and it would return a list of tags for the page. I did come across many but none were in a form to be used directly.
Here is a YAHOO pipe that generates Tags when a website is mentioned. I use the Tag Cloud Seeder page to generate the tags and then parse it using pipes.
The parsing has to be a little more streamlined and I am currently working on it. Once this service is done, it would be the way to generate tags for the ubiquity command.