Apparently a lot of HelpDesk software has this feature. Perhaps some trouble-ticket software does, also.
In our admin forms, there are often standard issues we need to address. Instead of typing them each time, or cut and pasting from historical stuff we've saved, we need a table or menu of standard items which cause the text to be inserted in the msg to the user. This happens all the time with project approvals, but there are others, eg. request_idleinfo.
Of course this is true of testbed-ops mail as well, but we don't have a Web interface for that. I wonder if there's some special Web-mail interface that handles that.
