Buried in the comments of this post is a useful link to an online scanner for police and fire radio of both Somerville and Cambridge (and also Brookline, Newton, Waltham, Watertown, and Wellesley):
Wicked Local Rangecast Police and Fire Scanner
It requires Java, which takes a while to start up in my browser. You can go back in time up to three hours. This is useful if you want to know why you heard lots of sirens an hour ago.
ETA: Here's another Rangecast scanner, for Somerville, Medford, and Arlington (and also Malden, Melrose, and Winchester).
ETA2: And here's yet another, covering Somerville, Cambridge, Medford, and Arlington (and also Belmont, Lexington, and Winchester). This one's probably the most useful for Somerville residents.
ETA3: Another comment in the earlier post recommends RadioReference.com for scanner feeds.
If you know of other online scanners, especially those allowing you to go back more than three hours, please post comments here.
I've added these links to the davis_square profile page.