Online video site Hulu formally launched its premium streaming service yesterday and cut the price to $8 a month.

The service launched as a preview about five months ago for $2 more. Hulu said subscribers will receive credit for the difference on what they’ve already paid.

Hulu’s service offers back seasons of NBC, Fox and ABC and other TV shows, including “30 Rock.”

Hulu Plus, however, offers high-definition content and is available on a slew of Internet-connected devices, including iPhone, iPad and PlayStation 3.

The free portion of the site, which has less content, remains available only on computers.

Hulu — jointly owned by NBC Universal, Post parent News Corp., Walt Disney and Providence Equity Partners — had said it’s aiming at 2010 revenue of $240 million versus $108 million in 2009.