Does what it says on the tin: route Growl alerts to the 10.8 Notification Manager. Until most app devs get around to fully supporting Notifications, it's a good way to leverage all the Growl-enabled apps out there, without the need for Growl itself.
There are some limitations: all Growl alerts show up under one App group in Notifications. This isn't a bug; Notifications groups the alerts by app, and it can't tell which app sent the alert to Hiss. Not much the dev can do about that, it's an Apple architectural limitation.