Several suggestions.
() Apple Bluetooth Wireless Keyboard. Good feel. Light and compact without feeling cramped. Aluminum construction is very durable. Fits the Incase Origami Workstation that unfolds into an iPad stand. The combination is an alternative to the Zaggmate/Logitech (and some other) combo case, allowing the iPad to be used with or without the keyboard.
() Microsoft 6000 Bluetooth Mobile Keyboard. Excellent feel. Ergonomically curved and slightly wider than the Apple. Separate number pad available. Significantly less expensive than the Apple keyboard ($30-$45). Plastic frame but very durable. Flat and easy to pack but cannot be used with the Incase OW or any other combo stand/keyboard case as far as I know. If that doesn't matter, it's actually superior to the Apple in terms of feel and ergonomics, imo, of course. And at $30 for the version without a separate 10 key number pad, it's a very good deal.
() HP Touchpad Wireless Keyboard. This is the new keyboard introduced with the HP Touchpad Tablet. Similar in looks/design to the Apple keyboard but available in black rather than the Apple's aluminum/white. Same price as the Apple ($55-$60). Works with iPad flawlessly (according to the review below.)
MobileTechReview News, Reviews and Discussion: Tablet Keyboard Review: HP TouchPad Wireless Keyboard Review
None of these keyboards have iPad-specific keys such as "Home" (including oddly, the Apple keyboard.) However, all should support arrow keys in applications that provide that functionality. (Can't verify for the HP, but I have each of the other two and I suspect it's the same for the HP.) And each supports audio and brightness control.
I'm not a big fan of "folding" keyboards. I've found in the past that they don't hold up to extensive use and don't have the same feel as the options above. But YMMV, especially if you're not as picky about a keyboard's feel as I am and don't use it as much as I do.
You may also prefer a combination case/keyboard (e.g. Zaggmate). I don't simply because it means I must always carry a keyboard with the iPad and I can't move the keyboard away from the screen. Again, YMMV.
The Microsoft Keyboard (both versions) is available from
Amazon and buy.com. Both the Apple and the HP are available from
Amazon. The Apple can, of course, be purchased from Apple and I suspect the HP can be purchased online from HP. Haven't checked.