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I'm embarrassed to admit it, but probably AppAdvice -- so I can see which other apps are free that day, and download them. Of course, I rarely make the time to use most of them, but somehow it's comforting knowing they're there, if I ever do need them. As for apps I actually use, Safari is #1 for me.
 
Aside from the standard/built in apps (i.e. mail, safari), mine would be Baby Monitor. Live HD streaming (with infrared night vision) with audio from my home that I can view anywhere where there's Wifi or 3G. I currently have cameras setup in a couple of rooms in the house.

http://www.wifibaby.net

Even though this is targeted towards parents, it actually can serve as a pretty cool security system. It has motion detection and audible alerts that can record video and/or pictures that can be emailed to you in real-time or you can just have it email you pictures periodically if you want to see what's going on from time to time -- or better yet, just logon to the app where ever there's wifi or 3G (technically, it works with EDGE as well, but pretty much useless with that speed). The app also supports multiple camera monitoring.
 
I use the SlingPlayer App on my iPad a lot. Great for watching TV around the house or out in the garage via my home Wifi. Tonight I watched it outside on the back porch while I did some work. Often I will use it to watch TV while running on my treadmill, as the iPad sits nicely on the treadmill's book holder.

But most of the time I use the SlingPlayer App with my iPad sitting in landscape mode (in the ZooGue case), so I can watch the news while using my 13" MacBook Pro to surf the web.
 
iCab and AVplayerHD.

AVplayerHD alone makes the iPad a billion times better as an overall product.
 
Safari, Mail and the Times of London newspaper. The latter is really what an iPad periodical/newspaper should aspire to....
 
GoodReader, 75% of time, Zite 15%, rest 10%. Which means iPad is basically an e-reader for me.
 
To me this question needs two answers, Apple-supplied app and downloaded app.

Safari by far in regards to an app that came with the unit.

As for a downloaded app, I'd say the Time Warner Cable app now. Before that it was the IMDB app.

Mike
 
iReview

I used iReview for about 4 days straight while studying for a job interview. I'm glad somebody wrote this utility!
 
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