eSavers and Hot Seat Specials are listed at http://www.greyhound.com/deals/fare_specials.asp If you click the link for one of the listed city pairs, the Greyhound website will put the appropriate discount code in the appropriate box as you proceed through the ticketing process.

It's probably easier to keep an eye on http://www.greyhound.com/deals/fare_specials.asp than to consult a list of codes (unless someone knows of a Greyhound discount code for a fare that isn't promoted on the Greyhound website).