I named some of the biggest.
In terms of producers (or manufacturers, suppliers) of power generation technology, General Electric (GE, US), ABB Group (Sweden/Switzerland), AEG (Germany), Alstom (France/UK), Siemens (Germany), Mitsubishi, Hitachi, Toshiba, Doosan Heavy Industries & Construction, . . . . are some of the biggest.
Then there are the users of power generation/distribution technology - basically private/public utilities.
US - Exelon, Entergy, Progress Energy, Dominion, Duke (Power/Energy), Constellation, Xcel, AEP, Southern Co, TVA, National Grid, Edison Electric, Ameren, Dynegy, PPL Group, . . .
Some electrical utilities are listed as Utilities: Gas & Electric,
See -
http://money.cnn.com/magazines/fortune/fortune500/2009/industries/143/index.html
others are listed as Energy
http://money.cnn.com/magazines/fortune/fortune500/2009/industries/165/index.html
Europe - EdF, GdF Suez, E.ON, RWE, Vattenfall, Iberinco, . . . .
Asia - Tokyo Electric Power Company (TEPCO), Kansais Electric Power Co. (KEPCO), Korean Electric Power co (KEPCO), Taiwan Power Co, (TaiPower), . . . .