unique IDs for windows?

Do browser windows have, or can they be made to have, unique IDs that 
are accessible by URL response generation functions? I'd like a clean 
simple way to store some state that is specific to a window. I'd 
appreciate any pointers.
    Thanks,

        Mark Klein
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Mark Klein, PhD
Principal Research Scientist
Center for Coordination Science
Massachusetts Institute of Technology
3 Cambridge Center, NE20-336
Cambridge MA 02142

Tel:    +1 (617) 253-6796
Fax:    +1 (617) 452-3231
Email:  m_klein@xxxxxxx
WWW:    http://ccs.mit.edu/klein/