The most annoying of all American accents, just ahead of the Philadelphia accent. Most commonly heard from douchebag Red Sox fans who suddenly like to brag about the "Sawwwx" greatness, it sounds like someone cut out a chunk of their tongue, making it impossible for them to pronounce "r" when appropriate, and inserting the "r" sound into words where it doesn't exist. Extremely irritating, as is typical for Bostonians.
Example of a Boston Accent in conversation:
Sean: "Hey, wheah's Pawlie?"
Patrick: "I dunno, I just sawr him the uddah day at dah pahty. He was wicked drunk."
Sean: "Yeah, I sawr him theah too. He was pukin next to a pahked cah. Musta had a hawrrible hangovah the next day."
Patrick: "Yeah, I bet it was wicked. Hey, you going to da Sawwwx game dis Satahday?"
Sean: "Of couahs I'll be theah. Varitek is playing awrsome at cathca lately."