For certain values of &quot;mainframe&quot;, some are operational or being restored at the Living Computer Museum in Seattle (a very neat place).<p><a href="http:&#x2F;&#x2F;www.livingcomputermuseum.org&#x2F;The-Collection&#x2F;Exhibit-Hall.aspx#Mainframe" rel="nofollow">http:&#x2F;&#x2F;www.livingcomputermuseum.org&#x2F;The-Collection&#x2F;Exhibit-H...</a>