One of the most egregious examples of this phenomenon has happened in recent days. Round about the time the giant healthcare bill was passed, the media went out of its way to smear the Tea Party activists. The Tea Party movement represents a threat to the Democrats and their plans, so there was a concerted (though not necessarily coordinated) effort to stigmatize the entire movement as either extremist, violent, racist, or all three.
There are three articles in particular that I have read recently that show how this smear attempt has been carried out. I consider them extremely important reading if you have any doubts about what you have been told by the media regarding the Tea Party.
First is a very measured article by Michael Moynihan writing for Reason.com about how several well-known journalists have taken positions comparing Tea Partiers or conservatives to Nazis.
Second is a great summary by Andrew Breitbart about the suspicious 'n-word' incident at Capitol Hill really went down. He's been great at exposing the strategies and tactics involved when the left attacks.
Finally, John. M. O'Hara describes how the media hyperinflates any small incident or sign carried by a lone nut at a Tea Party rally (as there inevitably must be at such large gatherings), yet overlooks similar whackos on the left who showed up at anti-Bush/anti-war rallies of yesteryear.
As a small bonus, if you want to see a great video of a Tea Party event, and really see what I believe is representative of most Tea Party attenders, check this one out.