One of the best (in the sense of "most comical") regional dialects of American English, native to eastern Massachusetts. basically, any "ar"/"er" sound is replaced by "ah". In semi-technical terms, the accent is characterized by loss of the post-vocalic 'r' sound, e.g.:

car     -> cah
mother  -> moth·ah
garbage -> gah·bij

also: 'wicked' can be used as an intensifying adverb (like 'very') and a carbonated beverage is either called 'soda' or 'tonic', depending on location and the age of the speaker. Under no circumstances is it referred to as 'pop'.

A tricky accent to fake well; Robin Williams's attempt at Boston English in Good Will Hunting was laughably bad.

Even some people who live in the suburbs of Boston have a difficult time getting the accent right. For example, where I grew up (25 minutes from the city by highway), people do not have a strong Bostonian accent. Throughout elementary school everybody spoke with a very standard American english accent. In fact, we used to think that my parents, who are from Boston, spoke very oddly and in general laughed at kids with Boston accents.

However, when we hit junior high school, we went to the regional high school, and there were cool kids from the neighboring town, which was a little less upscale- the "cool kids" from that town had Boston accents. And so throughout junior high school there was this phenomenon of hearing kids from my town whose names are things like Jessica Smith or Robbie Herrick, really white-bread waspy suburban kids whose families were not from Boston to begin with, who for the sake of coolness went around all day speaking with laughably poor Boston accents so that they could be accepted with the kids from the other town.

Some more quirky Boston pronunciations (a lot of which are dying out with kids of my generation):

• mine is pronounced "mayan"
• Filenes (a department store chain) is pronounced "faleans"
• potatoes is pronounced "puddaydiz"
• going is pronounced as "go win"- in fact the "g" on many words ending in "-ing" is often not pronounced
• "can't" is sometimes pronounced with a short "a" rather than with a longer "a" as in standard american english. In other words, as riptor pointed out below, some words have a pronunciation closer to British English than to American English.

I have to say that probably the current Boston accent is a mixture of the english spoken by the earlier english settlers of the area with that of later Irish immigrants(I base this on the fact that my grandmother, who is Irish, as well as a lot of the other Irish people I know, speak the most "ideal" Boston English) but this is a subject that definitely merits further study.

I too am currently growing up in a northern (nawthin) suburb of Boston and yes, this conversation has come up many a time between me and my peers. In my town, a Boston accent is rare among the younger generation, but, like wukong888 said, common among their parents. My mom, for example, grew up in Somerville, and regularly talks about tawnic (tonic) and taking bahths (baths. Pronounced almost London-like).

However, I would like to clarify some things:
Number one: If you went to Harvard Yard (yeah yeah pahk'd yeh cah theiyah), you could very easily order a soda and receive a Coke or Mountain Dew or the like. We're not so insular to consider drinks of that ilk to only be "tonic". I call them sodas myself

Number Two: On The Usage of "Wicked": Using "wicked" as a synonym for good is a skill, an art, if you will. We don't say wicked good that often. The word is best used in passing, said causually. If you come flying out like gangbusters with something like, "HEY GUYS! WICKED NEAT GAME THERE HUH?" you will most likely be looked at oddly. Same goes for looking at something interesting and proclaiming "Wicked!". No one does that. Wicked is meant to be used as an adjective, not an exclamation. The most common use of it (nowadays, anyway), that i have noticed is "wicked sweet", i.e. "Yeah i know, it was wicked sweet". There's a certain level of apathy and subtlety that it needs to be said with. Well, we've got that cleared now.

Plus, there are several regional accents: Boston(Bawstin), Somerville (Summihvil), Revere (Reviyah), Medford (Medfid), Billerica (B'ricca), Worcester (Woohstah), and Chelmsford (Chemsfid) all have discernable differences.

For the record, you cannot pahk your cah in Hahvid Yahd.

Unless you are a Hahvid cop.

And it's pronounced "Hahvid" in good B.E., not "Hahvahd", unless you are a Boston Brahmin, the now-rare onetime Boston intellectual elite.

For more info on Boston English see The Wicked Good Guide to Boston English: http://www.boston-online.com/glossary.html