Sigma has been giving Canon and Nikon a run for their money in their recent introductions. Their 18-35mm f/1.8, 135mm f/1.8 and the 120-300mm f/2.8 lenses are well regarded, received great reviews and priced very competitively.
Now word is Sigma is ready to challenge Canon again in a very popular focal length. They are working to introduce a 24-70mm f/2 lens for full frame sensors. Personally, I find the Canon EF 24-70mm f/2.8L II lens overpriced ($2,299) in light of the incremental improvement over the Series I lens. Sigma's introductory time frame for their 24-70mm lens will be mid 2014. Watch for the price of the competing Canon lens to drop as 2014 approaches.