There are a bunch. Some of the most popular regular ones are:
Venture Cafe - Every Thursday there's open networking, usually with a featured guest or two from out of town, at the Venture Cafe space inside the Cambridge Innovation Center (the densest cluster of startups in the Boston area).
Web Innovator's Group - Each quarter WebInno throws a huge mixer inside the Royal Sonesta hotel. Hundreds of attendees from the VC & startup community attend. 3 'main dish' startups present demos to an audience, and six more 'side dish' startups have booths attendees can stop by to checkout. The next one is March 3rd.
Tech Tuesdays - A newer event happening regularly at Kingston Station in downtown Boston. General mixer for startup folks and techies.
Outside of the regular ones, there's almost always something happening. Greenhorn Connect and Venture Fizz both have event calendars that are helpful in seeing what's going on. The best way to stay informed is to be active on twitter -- good events are always tweeted / retweeted about.
I run Boston Startups Guide, so if you're looking to connect with any specific parts of the local scene, please feel free to drop me a line.