nbc is not biased, and because of that i cant provide you with sources. however, if you really want to see proof, turn on cnn and nbc. watch them both for half an hour. they tend to be centered and cover things in a similar way. then watch fox and msnbc, where you will of course see a right and left lean, as both are deliberate.
the accusation that the media is a liberal organization annoys the hell out of me. i think a better way to phrase it is "progressive", but is criticized as being liberal because "progressive" is kind of the opposite of conservative. you need to view it in the grand scheme of things, over the course of american history. the media, for hundreds of years and not just in america, has been fighting against being suppressed. leaps forward are what its about, and since editors are generally well-educated and are inundated with stories and must decide what has merit, they pick whats new, and leaps forward are just that.
the media is a watchdog, and the watchdog protects people. abolitionists, civil rightests, gay rights supporters, women's rights supporters, they all have a history of being tied with the media. the media is about moving forward, and securing rights and changing to meet whats best for the country is part of that.