Maphack was an illegal program in wide use by Diablo 2 players. Maphack allowed players to see the entire map as soon as they entered an area, rather than having to explore the area first to discover the layout of the level. Some versions of maphack would also show the location or full equipment of other players, as well as showing all of the monsters, shrines, chests, and more on the level.
The use of maphack violates the EULA and ToS, and numerous players have been suspended or banned from Battle.net for using it. Despite this, its use was (and is) quite common on the realms.
Purist players do not use maphack and tend to cheer on disciplinary actions taken against those who do. On the other hand, many players who use maphack argue that it's not "really" cheating, since it just provides a sort of shortcut through boring levels they've already played a million times during their Diablo 2 career.
It's not known if or how Blizzard will attempt to prevent a Diablo 3 version of maphack from being put into use. Like all games, Diablo 3 must send the user's client more information than is seen on the screen, to save loading time and reduce lag. From this it's simple for programmers in the community to find ways to unlock that information and gain access to it before they could in the game. This means that programs like maphack are virtually impossible to stop, and a company can only work to detect and ban players who use them.