I've been using the ones made by Shout and I can assure you they work ... very well.
I recently bought 5 1/2 yds of red fabric to use as backing on a Quilt of Valor. When I washed it in hot water, I threw in three Color Catchers because I knew it would run. Sure enough, when I removed the Color Catchers, they were bright pink.
I use them all the time now.