Try Linkedin Event Search: http://events.linkedin.com

Other sites:

http://conferencehound.com/

http://allconferences.com

Scientific and engineering:

http://physicsworld.com/cws/events

http://www.st.ewi.tudelft.nl/~mathijs/conf/

http://www.conferencealerts.com/engineer.htm

http://www.medscape.com/medscapetoday/conferences