IOS Browser Masquerading
In recent versions of iOS you can request a desktop site in Mobile Safari.
Long-press/hold the reload control icon (see annotated screenshot below) in the address bar until a dialog pops up, then select "Request Desktop Site".       

The user agent changes from iPhone OS (e.g. mobile OS) to Intel Mac OS X (e.g. desktop OS).

(Screenshots below show a German user agent check webpage, but you get the idea.)

Mobile Safari: "regular" iOS user agent


Mobile Safari: desktop Mac user agent after "Request Desktop Site"