Sigma has a very successful Art line of lenses for Canon and Nikon cameras. They are planning to match almost all the high end Canon glass on the market, including the Super Telephoto line.
They have a 14-24mm f/4 DG HSM lens available now but is planning to announce the f/2.8 version some time this year. Sigma is the better of the two third party lens manufacturers. Read my earlier post on the interview with CEO Kazuto Yamaki. Check back often for the latest information.