Just did a quick Google and found in Wikipedia

On a few occasions, the B-side became the more popular song. This was usually because a DJ preferred the B-side to its A-side and played it instead. Examples include "I Will Survive" by Gloria Gaynor (originally the B-side of "Substitute"), "I'll Be Around" by The Spinners (originally the B-side of "How Could I Let You Get Away"), "Maggie May" by Rod Stewart (originally the B-side of "Reason To Believe"), "Single Ladies (Put a Ring on It)" by Beyoncé (originally the B-side of "If I Were a Boy"), and Reddish -Diva Version- by D'espairsRay (B-side of their single "Gemini"). More rarely, both sides of the single would become hits, such as Queen's "We Are the Champions" and "We Will Rock You".

The entire entry in Wikipedia is an interesting read

http://en.wikipedia.org/wiki/A-side_and_B-side