For those of you with affiliation to Harvard Medical School, you can have access to the Harvard U athletic facilities (in and around Harvard Square) for $20 per year (postdocs/staff) or free (grad students). You must have a Harvard ID card - see Sandi for info on how to get that.
http://www.gocrimson.com/SportSelect.dbml?DB_OEM_ID=9000&KEY=&SPID=11955
Click on Membership, then How to purchase. Also see Locker & Towel service info. For locations and details, click on Facilities.