I thought I'd pull this post out from a YYZ thread as this question keeps coming up. Most consolidators won't sell retail so you still need to use a TA (or be one). Some TAs, usually chinese, offer even lower fares, so look around.

http://www.gttglobal.com/BasePage/branch.aspx
http://www.cnhintl.com/

All US consolidators:
http://usaca.com/airline-consolidators.cfm

Canadian consolidators:
http://www.skylinkcanada.com/
http://www.intair.com/en/index.php