Merriam-Webster's Word of the Day for June 05, 2009 is:

kibitzer • \KIB-it-ser\  • noun
: one who looks on and often offers unwanted advice or comment; broadly : one who offers opinions

Example sentence:
Sybil warned Jack not to take any advice from Carl, a notorious kibitzer whose suggestions often did more harm than good.

Did you know?
The Yiddish language has given English some particularly piquant terms over the years, and "kibitzer" (or "kibbitzer") is one such term. "Kibitzer," spelled "kibitser" in Yiddish, came to that language from the German word "kiebitzen," meaning "to look on (at cards)." "Kiebitzen" may or may not be derived from a German word for "lapwing," a type of bird noted for its shrill and raucous cry. (We can speculate that the bird’s cry reminded people of the shrill commentary of onlookers at card games.) The word became more popular and widespread after the 1929 play The Kibitzer came out. Although "kibitzer" usually implies the imparting of unwanted advice, there is a respectable body of evidence for a kibitzer as a person simply making comments.