According to a Nielsen year-end entertainment scan (via PC World), Sony's PlayStation 2 was the most played video game console of 2008.
Almost 32% of console usage minutes were spent on the last-generation system, close to double the amount of time spent on the Xbox 360 and Nintendo Wii. According to PC World, the PS2's advantage dropped from about 42% in 2007. Here's the complete list:
PlayStation 2 31.7%
Xbox 360 17.2%
PlayStation 3 7.3%
The Nielsen scan also finds World of Warcraft finished 2008 as the most-played PC game, followed by Call of Duty 4: Modern Warfare and the first Halo.