Ergonomic mousepads are made of a variety of materials and have many different design options. The first step in ergonomic mousepads was gel wrist rests. Soon after, mousepads became available with built-in wrist rests. That type is typically made with an oval or round gel-filled bump for your wrist attached to an average looking mousepad. Since those first mousepads came out, a number of new options have been developed.
Memory foam mousepads are exceptionally comfortable. Some options simply replace gel with memory foam and appear very similar to the early ergonomic mousepads. Other memory foam mousepads, like the Ergo-Mat, don't have a separate wrist rest. Instead, they are a thick pad that's sloped, so you can rest your wrist and hand on the pad.
Another version of ergonomic mousepads is the lap pad mousepad. These mousepads have a sort of bean bag attached to the bottom, so you can place your mousepad in the most comfortable place for you. Not only can you put it on your lap comfortably, but also your arm rest or keyboard tray. The Flite Tech mousepad also sits on your lap, but is designed specifically for use on an airplane.
In 2006, the Shouldercool Mousepad was introduced. I'm guessing you never heard of it. If so, you're not alone. Not all ergonomic designs end up being as practical as they need to be for daily use. The Shouldercool is an important reminder; when you are shopping for ergonomic solutions, the newest and weirdest looking solution is probably not the best one.