Yakima Mako Aero Saddles
The Mako Aero saddle clamps on to most kinds of bars, and the installation is easy. They are sold in a pair, but two pairs are needed to carry one kayak. One for the front bar, and one for the back.
There are also some nice extras included in the box. There is a tie-down strap with rubber covering the cam buckle, which protects both your boat and the roof of your car. Also included is a bow or stern tie-down which hooks the end of your boat to the car's tow hitch. And each saddle comes with optional felt padding, to protect the boat. I found that with the included straps, nothing additional was needed to secure a kayak.
I've tried both the Mako saddle and the Mako Aero saddle. The main difference is the Mako Aero is compatible with more kinds of bars, such as the Whispbar. Also, the Mako Aero is compatible with a locking core, which isn't available for the regular Mako.