N612UA, which operated United 175, was seen in both New York and Los Angeles during 2001:
http://www.airliners.net/search/photo.search?regsearch=N612UAN334AA, which operated American 11, was spotted in both Chicago and Stockholm during 2001:
http://www.airliners.net/search/photo.search?regsearch=N334AAN644AA, which operated American 77, was spotted in both Miami and Boston during 2001:
http://www.airliners.net/search/photo.search?regsearch=N644AAN591UA, which operated United 93, was spotted at Newark on September 8th 2001:
http://www.airliners.net/photo/United-Airlines/Boeing-757-222/0188153/L/And that's just from one photo-site, I wouldn't be surprised if you could find more on other sites, especially those accepting lower standard images.
Also, here's a fun exercise (hat tip to the JREF crew):
Go to the BTS inquiry page. (
http://www.bts.gov/xml/ontimesummarystatistics/src/dstat/OntimeSummaryDepatures.xml)
Check "All statistics".
N612UA regularly operated Flight 175 out of San Francisco, so choose SFO as the Origin Airport.
Choose United Airlines as the airline.
Check September for the month.
Check All Days to see information for all of September.
Check 2001 for the year.
Click Submit. And wait (that's a big query).
And lo and behold, N612UA is shown as having operated out of San Francisco at least 4 times during the month of september.