Fat Gripz were the first brand of adjustable thick grips that came out over 10 years ago. In strongman, having a good grip is important and typical strength equipment comes in various grip sizes i.e. 2", 2.5" and 3". Buying grip training tools in the form of fixed dumbbells etc can be expensive and take up a lot of space. So when Fat Gripz developed the mobile, adjustable grips which could be attached to mostly any dumbbell handles, barbells, machines, chinning bars etc, it proved to be highly successful.
I've owned Fat Gripz for years now and love them. Getting away with just using a standard dumbbell handle for example, A Hex Rubber encased Dumbbell can easily be made to feel so much heavier by adding Fat Gripz! If you don't believe me, try it.
Fat Gripz currently come in three different sizes...
Fat Gripz One - 1.75" (4.5 cm)
Fat Gripz Original - 2.25" (5.7 cm)
Fat Gripz Extreme - 2.75" (5.7 cm)
I highly recommend them as there great for keeping training interesting but more importantly, they work in producing stronger grip strength and forearm size.
Other thick bar products on the market include Iron Bull Strength Alpha Grips which are also very good and have a slightly different design.