Notify.io
Status update: Official Notify.io support has been discontinued due to instability of their service.
Notify.io is a service that lets you get desktop/mobile notifications from web applications. It's free to sign up, and easy (as long as you have a Google account).
To set it up with Tender, pick a discussion creation/reply event. You can either enter the email that you sign into Notify.io with, or the User Hash that Notify.IO gives you (see the screenshot above). Don't use your API key.
Notify.io is completely free of charge and open source. It has small desktop clients on the Mac and Windows that display your alerts as they roll in.