BncWeb - http://bulba.sdsu.edu/bncweb
MicaseSearch - http://www.hti.umich.edu/cgi/m/micase/micase-idx?type=revise
LinguistsSearchEngine - http://lse.umiacs.umd.edu
TalkBank - http://www.talkbank.org/
VariationInEnglishWords - http://view.byu.edu/
WebCorp - http://www.webcorp.org.uk/
Corpus of American English (a very large and continually updated corpus- 385+ million words)- http://www.americancorpus.org/