I was chatting in IRC with some of the Rootsweb staff, and found out
about an unlinked search engine, which searches all of the USGenWeb
archives for the entire US. Naturally, this is only valuable for rare
surnames or well put-together queries, but
CAHOON/CALHOON/CALHOUN/COLQUHOUN is pretty easy!
http://searches.rootsweb.com/usgenweb/fullusaindex.html
Calhoun yields 6508 hits, Calhoon 557, Cahoon 458, Colquhoun 48, so the
state-separated engine would probably be better:
http://www.rootsweb.com/~usgenweb/ussearch.htm
Have fun!
Valorie