If you're wife is into technology then the iPad is the most popular item around. Just be sure to get the version that has 3G connectivity or she'll be dependent on a WiFi connection. Other technology gifts include either a new iPhone or Blackberry.
If you're wife is a blogger, loves to scrapbook or is pretty much in charge of documenting the family, then other gifts include a great compact digital camera (so she can have it with her at all times) or a Flip compact video recorder.
IPhone & IPad are great ideas if she is a fan of Apple products. A new digital camera is always a great idea too if she needs one. If she's a big reader you can't go wrong with the Kindle & an Amazon gift card for a few new books or a cover. I love mine!
how about taking her to a nice dinner date without telling her where you're going. find out if any of her favorite band or artist is in town and surprise her with that? Time with your wife is more precious and unforgettable than a gadget she can get on her own :). make it a romantic night if you haven't done it in a while.
Ugh, what if your wife is totally not into technology? This is my second referral to gifts.com. Use it to figure out what kind of gift your wife would be into by answering some basic questions about her (that hopefully you can answer).