Canopus is a major competitor to Sorenson, but my own experience with it was that, compared to Sorenson, it was clunky and it seemed to be a resource hog.

They may have cleaned it up since then, I last tested it a year or two ago.

And I am having some weird issues with Sorenson (4.5) right now, but I think it's due to some update that Microsoft pushed to XP, because it was working on the two machines we have it installed on, and all the sudden it broke.