Turner Motorsport (for the hard stuff or high precision, I would go here. You can't see your car being worked on though, so that's a real turnoff). $$$$$$
Mighty Motors in Somerville (Jim is gone though, sadly.. ask for Sam, younger dude, nice guy) $$$$
Mike's (?) .. there's another new place in Somerville.. the mechanic there is a prestigious race mechanic, the details elude me, never been there. PM me if you want me to look it up.
Landshark in Natick is recommended by some, and I know someone who goes there. I've never used them. $$$$$