Hi, and welcome to JA!
The most popular place that people who are looking to rent an apartment go is Craigslist, so that should be the first place you post your listing. Here is a link:
(it will end up on this page: http://boston.craigslist.org/aap/)
Craigslist, however, will get you the best results.
There are also non-free options. Let me know if you are interested in any of those.
I hope this helps,
Here are some non-free options:
Here is a complete list of Boston-based agencies:
Let me know if I can help you further, and I certainly will!
Thanks for the accept, feedback, and wonderful bonus!
Did you need any further information, in addition to that complete agency list (http://www.bostonapartments.com/agencies.htm) and the other sites I have given you?