Complete the author, email & url fields (& focus on the message box) in most comment forms by hitting 'ALT+C'
It is designed to work with wordpress blogs, but will work with any blog that uses the same id's for fields.

You will be asked for your name/email/url the first time you run the script. It will then remember your values so you never have to fill them out again!

To change your name/email/url at a later date just hit 'SHIFT+ALT+C' and you will prompted for your new values.

If you don't like the default keyboard shortcuts you can change them under:
Tools > Greasemonkey > User Script Commands > Set Prefill Comment Shortcuts