PIGG is the file extension type used by the "City of" game line. The extensible file format is used to create virtual drives for a software program, in this case, cityofheroes.exe
Through the use of some third party applications, some players have been able to look into the PIGG files, to extrapolate information from the game, such as hidden badges, yet-to-be-released content, and so forth. This activity has come to be known as "pigg diving."
Although there are freely available programs for viewing PIGG files (and therefor being able to search the game files themselves for any game content, regardless if it is active or not), discussing the details of such activities can constitute "reverse engineering" of the game and violates the End User License Agreement:
You may not (a) sublicense, rent, lease, loan or otherwise transfer the Software for profit; (b) modify, adapt, reverse engineer or decompile the Software, or otherwise attempt to derive source code from the Software; (c) create any derivative works in respect of the Software or the Service;
Because of this, the public discussion of details on how to view PIGG files, PIGG file contents, etc. is considered taboo among the "City of" community, although it is understood that some players perform such actions as a way to see "spoilers" on locked or upcoming content, to find specifics about certain game mechanics, or other non-profit tasks.