Boston
The following is a compendium of Boston-area agencies with a web presence. There are others worth considering (e.g., Premiere) that do not have a web presence, but I had to stop somewhere. Also, note that my comments include hearsay from known and trusted posters and backchannel friends! [the required "Lubedrod" admission...] 
There are a number of primarily incall agencies with good reputations...
Genuine Diamonds... http://ninasgenuinediamonds.com/ Nina focuses on northern MA and southern NH, small service but very dependable. You must email Nina the first time.
Room Service... http://roomservice.newengland-escorts.com/ Jill provides incall all over the Boston area. Lower rates, half hours available. Generally dependable, variable quality.
Body Oasis... http://www.bodyoasis.com/ Amber started with American Beauties 2, changed the name and built up a reputable and sizable operation. Recently she dissolved Body Oasis to focus on Blue Pleasures, an on-line "adult community"... The ladies from Body Oasis scattered -- some apparently retired, some went indy, others reformed into the following two agencies...
Working Girls... http://www.workinggirlz.com/
Eastern Companions... http://www.easterncompanions.com/
Adara's Sweets... http://www.visitinggirlfriends.com/ As far as I know, Sonny's operation is quite new. He seems to have a strong pipeline to Brazilian ladies and (although I don't know personally), they are well-regarded...
The BellaDonna... http://thebelladonna.com/home.html Although it is local, this agency focuses on importing visiting ladies, more expensive and generally GFE.
The following two agencies offer both incall and outcall. They are both relatively new to the web (but not necessarily to the business). Both seemed dogged by spotty reputations. Experiences seem to range from very positive to downright abysmal...
Dangerous Curves... http://www.dangerouscurvesusa.com/pages/1/index.htm In her early postings, Taylor seemed to believe that belittling the TBD and TER posting community would win her business. Fortunately, that raw anger seems to have subsided. She also gets the award for the worst web site of the Boston adult community...
Exotic Pleasures... http://www.skyy.123ts.com/ In contrast to Taylor, Mike is openly solicitous of clients and saves his public ire for the escorts who leave his site...
The following agencies are exclusively outcall (to my knowledge). Their reputations vary...
After Dark... http://www.afterdarkescorts.net/index.html Holly's been around the industry for a while and runs a tight, fairly small, and low-key operation. She seems to do very well finding young attractive GFEs...
Friends in Boston... http://www.friendsinboston.com/index.html This agency is quite new, and I believe Milton is fairly new to the business. They offer high-end ($300/hour) ladies... early reports appear quite favorable. They would also get my vote for the best agency web site in the Boston area...
Temporarily Yours... http://www.tempu.com/ TempU has been around for a long time, quietly holding down the mid-range of the agency outcall market. I've never used them, but my general sense is that they are pretty dependable, have some attractive ladies, but don't really cater to the GFE end of the market...
Blue Moon... http://www.bluemoonescort.com/HOME.htm Blue Moon is affiliated with TempU and offers basically the same ladies (but without the web site pictures). Used to be that Blue Moon prices were lower, so those in the know would check TempU for the pictures, then call Blue Moon. I think that loophole is now gone...
Undressed4U... http://www.undressed4u.com/ This agency primarily works southern/central NH, but it's such an egregious rip-off that it's worth being aware of it. Again, I've never used them, but the reviews are almost uniformly horrible (not just bad!)...