A standalone keyboard must be carried separately from your iPad, and it usually requires you to buy a separate iPad stand (or a case with a built-in stand). But standalone keyboards offer a number of advantages over other types of iPad keyboards--most notably, standalone keyboards often use full-size, high-quality keys arranged in a standard layout, and when you don't need the keyboard, you can easily leave it behind to travel light. A standalone keyboard also lets you use your favorite case, and it works with any iPad, iPhone, or iPod touch. (Most can even be used with a Mac in a pinch.) For these reasons, this is my favorite type of iPad keyboard unless you frequently need to type on your lap--a task for which an integrated keyboard/case combo is better suited.
Logitech's Tablet Keyboard for iPad ($70) sports great keys, feels rock-solid, and comes with a hardshell keyboard case that flips open to double as an iPad stand. However, the keyboard is relatively heavy--with the case it weights 21.6 ounces--and, like the Logitech Fold-Up Keyboard recommended above, it omits dedicated F-keys in favor of fn-key-modified numeral keys. Targus's Targus Bluetooth Wireless Keyboard for iPad ($64) features large, easy-to-press keys, with a dedicated row of special-function F-keys and even an fn-key-activated embedded numeric keypad (like the one on many older PowerBooks). While the Targus keyboard isn't as solid-feeling as the Logitech model, it makes up for it by weighing just 9.6 ounces--three ounces lighter than Apple's Wireless Keyboard (below).
Honorable mention: Genius's LuxePad 9000 Ultra-thin Bluetooth Keyboard for iPad ($60) isn't quite as solid as the Logitech model, and it isn't as light as the Targus keyboard, but it's a quality product that's available for just over $43 on Amazon.com, making it a nice budget option.