Some polls are more accurate than others. Nate Silver grades them here. A few of his ratings of well-known pollsters:
A+ … Monmouth, ABC/Washington Post, Field, Elway
A … SurveyUSA, Marist, Farleigh/Dickinson
A- … Ipsos, Fox News, NBC/WSJ, Quinnipiac
C+ … Rasmussen, Harris Interactive
C- … SurveyMonkey
*F … Research 2000, Strategic Vision
*To earn this grade, you have to fake your results or engage in unethical conduct