Leftists have yet to present good objective evidence of a right-wing tilt of the mainstream media output. Their idea seems to be that if a business owns it, it must be conservative, by the odd assumption that all businesses are conservative. Ignored are the people actually generating the news, which are liberal/Democratic by large margins.

Surveys here,

here (compare journalism results with general public),

and a study here.

Sometimes it's just a matter of degree. Many radical leftists are convinced that the press is a lackey of the Right simply because the White House press corps doesn't leap on Bush when he enters the room, rip out his heart, and eat it raw.

Incidentally, I don't ignore mainstream media as a source of news. How many times have I linked to stories from those source? I just don't assume they're as objective as they claim to be, and take it with the same grains of salt as proudly biased sources.