Dropkick Murphys (Band), specifically one song, "Tessie", which you can watch them play here
Reasons for like it so much... (in the form of a history lesson)
1) Way back in 1903, "Tessie" was played during the first ever world series
, which the Red Sox won, over Pittsburgh.
2) In the subsequent Red Sox World Series victories, Tessie was played
3) Coincidentally, when "Tessie" stopped being played, the Red Sox stopped winning world championships
4) In 2004, 86 years after the Red Sox won their last championship
, a band known as "The Dropkick Murphys" preformed a new version of "Tessie"
5) We all know what came next: The Red Sox Won!
"Tessie" came through in the clutch, like David Ortiz
Yeah...that is why I like it so much...it inspires the Red Sox, and (in the Case of the 1903 Pittsburgh Pirates) caused their opponents to lose. (and is a great song, which you can still watch here
(shorter version - no band intros)