I'll Try Update as often as I could, hope you iPhone user out there can PM me or reply with some recommended app plus little review.

Facebook
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Newly Updated version 3.0, provide new "9 square" landscape interface to provide user easier to access the menu and notifications.
Additional new features in the 3.0 update include the opportunity to upload video direct from the iPhone 3GS, upload and manage photo albums, including creating and deleting albums and photos along with tags and changing your profile picture.

Openrice
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One of the best restaurant finder app for Hong Kong. Together with iPhone's GPS, u can find all the restaurant featured on the Open Rice website near you. Or search by location or type of food on the menu. The updated reviews and ratings are also provided in the app.

Palringo
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Some voted as the best non web based Instant Messenger app for iPhone. one universal IM that works with window Live, yahoo, AOL, Google talk, ICQ, Jabber, iChat/mobileMe, QQ, Gadu-Gadu and facebook chat.
Easy to use. But need to setup a free palringo account before you starts.
One thing I noticed is that , other user of Win Live Messenger don't see your status as using mobile device. (correct me if i am wrong)

Dictionary.com
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Their website is my first choice when I want dig up some english words. Their app is also quite good, keeping the simple structure and the voice pronunciation. But it lost the multiple dictionary result, in which some professional words could sometime can't be found in their own database.

iTranslate
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If you love google translate (i do) , you will love this, this app inter graded with google translate but can't translate webpages like the original.