Sigma Teleconverter 1.4 and 2.0
I think the Sigma TCs work only with Sigma lenses.
Anyway I had their 2X TC with my Sigma 70-200/2.8 and didn׳t like that combo at all.
Later on I sold the TC and got the Kenko pro 300 1.4X, worked with nearly all my lenses and gave great results.
Haven't tried their 2X version though....