Postdoc listserv: sign up at http://web.mit.edu/mitpostdocs/listserv.html
Information Guide for Postdoctoral Scholars
The MIT Postdoctoral Association
The Postdoctoral Mentoring and Advising Toolkit includes forms for use or adaptation by postdocs and their advisors (an MIT statement on the postdoctoral period, an outline for an initial meeting between an advisor and postdoc, mentor plan templates, annual review forms, a training checklist, and more)
New England Drug Metabolism group: job postings; regular meetings attended by drug company scientists and researchers from local universities
Boston Area Mass Spectrometry Group: career networking opportunities at regular seminar meetings