This is a modified version of the script at http://userscripts.org/scripts/show/12874 It uses underscores instead of dots, so it produces email addresses like domain_com@yourdomain.com instead of domain.com@yourdomain.com

A script to insert an email address like 'youraccount+currentdomain@gmail.com' so that you can easily filter your Gmail, track who's sending you spam, block senders if necessary. Press "F9" if you're in a text box to insert the email address.

A script to insert an email address like 'cnn.com@mydomain.com' so that you can easily filter mail, track who's sending you spam, block accounts if necessary. Press "F9" if you're in a text box to insert the email address.