Back in October, I had a member ask us if we had space in San Francisco.  I told them that we did not, but, I did refer them to use Citizen Space.  I exchanged an email with Tara Hunt at Citizen Space and she told me that they “have a free drop-in policy…so anyone who comes from thinkspace to SF to hang out for a couple of days is welcome at Citizen Space.”
The Coworking Wiki site has something called a “Coworking Visa“.  This allows active members of one space, when traveling, to use another coworking space, gratis. Terms vary from space to space, with regard to hours of operation, reservation requirements, etc. Those currently listed on the Coworking Visa page agree to offer up to 3 complimentary, drop-in days, with guidelines listed about using those days.
After running around Portland the other week, I stopped off at Souk and Cubespace.  Those are two other Coworking places in Portland.