There are numerous options available. Logitech has a couple of recent entries with very good keyboards. Zagg was the first major brand and several others are now available with varying levels of quality of the keyboards.
One caution, however. If you're accustomed to a full size keyboard, or even a laptop keyboard on an 11" notebook you may find the keyboard/case combos to be very cramped. If you're an accomplished touch typist, it can be a major problem.
If that is a problem for you, consider the option of a separate keyboard case/stand such as the Incase Origami Workstation. The price of an Apple Wireless Keyboard and Origami Workstation is approximately the same as the Zaggmate keyboard/iPad combo case.