Take a look at this recent thread: http://www.chowhound.com/topics/371633
Also, posters on this board often recommend Truly Jurg's in Kenmore Sq. I haven't been there but its gotten favorable reviews.
If you need to satisfy a sweet craving, Garden of Eden is a good bet - they have a decent variety of pastries and cookies. But, if what you need is an occasion cake, Party Favors in Brookline has tons of options.
We love cake in our house and I usually make it but when I can't and I need one, I get it at Icing on the cake in Newton. They have a chocolate cake with chocolate mousee filling that is so damm good. We are always excited when an occasion calls for a special cake to order. I think that their cakes taste perfect. Beautiful and tender crumb and the icing is wonderful.
I had a bit of a cake hunting weekend, as I'm trying to decide on a cake for my b-day. Here's what I found this weekend:
Party Favors - (tried the vanilla and chcolate) the cake itself was fine, but the icing was very shorteningly - that is to say, very greasy mouth feel.
Rebecca's (deche de leche) - so-so, not bad, not amazing.
Vicki Lee's - Lemon love and Sliver winter (raspberry) - both cakes suffer from the same problem, the icing is italian meringue buttercream, which is sniff tasting, but this may be my personal perference.
Athan's - Deche de leche and Jamaica (chocolate cherry), the duche was good, but the Jamaica was FANTASTIC. layers of dark chocolate cake, chcolate mousse and cherry with chocolate ganache. It's the winner.
Hope this helps!
Sweet Tooth Bakery in Southie (yes, Southie!) does a lot of mediocre stuff, but their chocolate cake is delicious.
Also, I hate to contradict a fellow hound, but everything I've ever had at Garden of Eden, including their sweets, has been terrible.
For our wedding a few years ago we got a dark chocolate cake with mocha frosting and a chocolate ganache coating. Very easily one of the purest distillations of concentrated evil I've ever had, and a few relatives have gone on enthusing about the cake five years after the wedding! It's a variation on a common Rosie's cake, and their carrot cake is killa also.