I just read an article in the Mass High Tech (MHT) weekly newsletter.  Here's a summary of the free locations that people have used.  If you know capacities, please tell me.

Cambridge area

Broad Institute

Microsoft's NERD (New England Research and Development Center

In the Burbs

Sun Microsystems (Burlington)

Foley Hoag LLP (Waltham)

Other (not sure of location)

Millenium Pharmaceuticals

Shire

Novartis

Public Libraries