Although it's not particularly relevant since excessive bias is bad no matter what side it leans towards, it really isn't fair to say that left-wing media is anywhere near as outrageous as the right's information machine. CNN doesn't deserve to be part of the discussion as it tries to be apolitical and does a good job of riding the center (despite the fact that the far right likes to pretend otherwise because it's good for their cause), but analyzing MSNBC and Fox shows vast differences.
Fox does a good job of using its "hard news" side to perpetuate and spread messages that their commentators fabric from loose pieces of evidence, where MSNBC's fact reporting is incredibly dull, often tasteless, and stupid, but not particularly partisan. If you compare the commentators at Fox and MSNBC you find a similar difference in extremes: MSNBC consists entirely of liberal editorializers with the exception of a strong moderate conservative voice Joe Scarborough (who I actually rather like) where the only liberal on Fox is routinely matched up against lions, lacks force of personality, and isn't particularly incisive (Alan Combs). In general whenever a liberal appears on Fox, they do a great job of putting them up against someone who has the tools to rip them apart, to the point that I don't know how any unbiased person can see it as anything but purposeful.
Ed Schultz is every bit as bad as the worst that Fox has to offer. I wouldn't be surprised if he actually was Rush Limbaugh in disguise -- both are large, boisterous men who make absurdly extreme claims on the fringe of ideology. Keith Obermann isn't much better than Ed, I really don't like his condescending, vicious attitude. From there things chill out quite a bit, Chris Matthews is a centrist lefty with a very strange, old-school point of view that is unlike anyone else in television. Maddow is surprisingly fair despite the fact that she's a devoted liberal, though she has her slip ups occasionally. Newer additions like Lawrence O'Donnel are harder to peg -- I don't know where he falls yet.
Compare the MSNBC line up against the Fox likes of Shaun Hannity, Glenn Beck, Bill O'Reilly, and the Fox and Friends crowd, and really there's no contest as to who's more extreme. This doesn't even get into radio which is dominated by right-wing personalities -- left-wingers have a very little media influence by comparison. Newspapers are dying and Jon Stewart hardly counts as a left wing commentator (and is quite fair with his commentary which does not moralize or preach ideology as much as it criticizes extremism on all sides), so only by painting timid outlets like NPR as liberal can you really create the perception of widespread leftist influence. Unfortunately, that never stopped the Republicans from trying, which is why they've been doing it for decades now. It really does prove that if you repeat something enough, people start believing it.
Now, if you want to talk the left as a whole, moving away from media outlets, then I'd say things are pretty much even. Liberals are every bit as bad as conservatives when it comes to twisting fact to suit their political opinions. Yeah, they say Obama is a Muslim terrorist bent on fundamentally transforming America, but many others said 9/11 was an inside job intended to give justification for invasion of the Middle East in order to enrich corporate profits for Haliburton and secure oil.
Insanity is not partisan.