Facebook: Interestingly, Facebook hasn't released a dedicated iPad app yet--but "Facebook Ultimate," a Facebook iPad app with no connection to those behind Facebook, has managed to crack into the top 10 paid apps in just two days' time. It's made pretty clear that this is an unofficial app, so once Facebook does enter the fray, they'll probably knock Facebook Ultimate way down the list.
Amazon: Amazon's got two official apps in the iPad App Store: one for their essential movie-and-TV listings property, the Internet Movie Database, and one for the Kindle. Though the Kindle may not have fancy-pants page-turning animations, it does have a catalog that positively dwarfs the iBookstore--450,000 titles to Apple's mere 60,000. Plus, most of the titles still hold to Amazon's cheaper $9.99 pricing. It's surprising that Apple even let the Kindle app into the App Store--especially since it's more established and even preferable to Apple's own offering. And the IMDb app is surprisingly excellent: it's easy to navigate with fingers, and it gives access to HD movie trailers, galleries, and even local showtimes.
Netflix: Possibly the killer iPad app, Netflix's entry in the app store is one of the most exciting iPad apps ever (granted, it's only two days old, but still). Stream Netflix Instant Watch titles over Wi-Fi anywhere you take your iPad, customize your queue, and take advantage of Netflix's unbeatable library. Until that rumored Hulu app hits the App Store, Netflix is the king of iPad streaming video.
Disney: Disney wasted no time in taking advantage of the iPad. The Toy Story Read-Along app was one of the keynote apps used to demonstrate the iPad's capabilities as a next-gen book reader, and best of all, it's available for free. The follow-up, Toy Story 2 Read-Along, costs $9--but you (or your kids) might be so hooked you won't even care. The apps combine bright picture books, games, videos, movie clips, coloring pages, and even sing-along songs.
Gilt Groupe: Private fashion sales site Gilt Groupe has created just about the most gorgeous shopping system I've ever seen. It takes Gilt's already-classy black and gold aesthetic and uses the iPad's pinch-to-zoom multitouch to create an incredibly smooth and simple experience. If you weren't already inclined to blow $300 on designer sneakers, this app might help you along that path.
Twitter: Of course Twitter is popular on the iPad, but the transition from small screen to bigger screen is trickier than you'd think. Upscaling an iPhone Twitter app is a disaster--text and buttons are too big, there's tons of wasted space, and you have that pesky pixelization problem. Luckily, a handful of really solid Twitter apps designed from the ground up for the iPad have hit the app store. There's Twitterific, a gorgeous app that uses popovers to great effect; TweetDeck, similar to the desktop version, which adds multiple columns; Sociable, which adds in Flickr, Facebook, Reddit, and Digg support; and even instant messenger apps like AIM and IM+, both of which integrate your Twitter feed. And some of the really big names, notable Tweetie and Echofon, haven't even entered the game yet--Twitter on the iPad is a very big deal.