It's a well-established major national polling company. "Reports by Slate Magazine and The Wall Street Journal found that Rasumussen Reports was one of the most accurate polling firms for the 2004 United States presidential election and 2006 United States general elections, respectively."
http://en.wikipedia.org/wiki/Rasmussen_Reports
Most liberal blogs complain that Rasmussen has a pro-Republican bias. For example, its poll of Bush approval rating tends to be on the higher end of all polls (38% in the most recent poll).