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I would probably say Pulse Reader would be my main one. In a matter of seconds I can scan through the headlines of 7 or 8 news sites I frequent. I check it at least every couple of hours and it makes my news reading much quicker.
 
Game changers for me:

The built in video player -- amazingly effective distraction for my toddler, especially in a restaurant or on an airplane. Games work well too.

IAnnotate PDF -- invaluable tool for work. I can mark up a PDF with notes, strikeouts, drawings, etc. and email the file to my marketing manager, who can view them just fine in Acrobat. Works great with finger or Pogo Stylus.

Also I finally bit the bullet and did the Virtual Memory hack. Works great! Why didn't I do this sooner?
 
If it has to be one

For me it has to be Omni Focus, coupled with the iPhone version. It really does allow you to stop being distracted by stuff that needs to be done. I used an Outlook based organisation system before, but this is MUCH better.

Evernote is becoming increasingly important for me too.
 
Everyday Edition
Trunk Notes
Goodreader
Neu.Notes
Sky Walk
Pages/Numbers/Keynote

Of course, Mail, Safari and iPod get constant use, but the above apps are the things that make the iPad more than big iPod Touch for me
 
I am studying for Step 1 and iAnnotate gives me quick ability to search for any keyword. It is a priceless tool for reference.
 
Definitely going to say our app Zen Viewer for iPad, which is a file viewer and reader.

But past that I would say:

1. Air Video: Rocks for streaming any type of file. I love to stream videos (mkv, avi, wmv) while on the treadmill. Makes it a lot easier to walk in place for 30 minutes like a hamster.

2. Twitter: Awesome UI.

3. Flipboard. Another completely inventive user interface.
 
Sky Safari.....with their app and cable, I can control my Meade ETX 125 telescope....amazing! I tap the celestial object on the screen of my iPad, and the telescope slews until it is view.:D
 
Funny enough. I also use the iPad for the same stuff some musicians posted earlier. Though for me, the top of my list is stuff like iBooks and Good reader. I tend to use my iPad on music stands instead of having to keep a bunch of loose papers with music on them. It's a whole lot less error-prone and much easier to handle. Not to mention, you can have a ton of music sheets captured on the iPad where stacks of papers quickly get unwieldy. However, the fact that you can turn pages without error and without worry of dropping sheets, is a godsend.

This works out because well, the iPad is the right size, and it can rest on a music stand unlike a laptop. I also keep music previewed on it as well. Due to a bi of multi-tasking, you can keep listening to stuff you need to practice while reviewing the written stuff, and or set lists. There's so many ways to cheat and review with this combination.

Also, in a surprising way, I also often depend on Pages and Numbers on it. Perhaps it's because I've already put in some documents which get frequent updates. And the ability to one button export them for review is very nifty since I do share those documents.
 
Flipboard, Safari, Twitter, Mail, YouTube for content feed.

Pages, Pages, Pages as my hobby is writing.

Others
- Apple standard photos, itunes, video, maps
- Goodreader
- Popplet
- Notes Plus
- Dictionary.com
 
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A306 Safari/6531.22.7)

Goodreader -- epic.
iTether -- don't have to pay AT&T for 3G
Boggle.
Wolfram apps
 
Lots of good stuff in here. My additions:

Byline (RSS)
Instapaper
TuneIn Radio
MLB At Bat
CloudReaders (bare bones app to read comic book [.cbr] files.)
 
Simple Notes
Evernote
Dropbox
Things
Instapaper
Angry Birds

With the exception of AB I also have all these apps on my Macs and my iPhone. I like integration.

Started playing with

Zen Viewer HD
Dropcopy

I was surprised to see so many votes for pages. I just don't see the iPad and Word Processor really being a match. The iPad just feels more suited to a good text editor. Still trying to decide between Notetaker HD, Notes Plus, Penultimate and Audiotorium.
 
Reeder for iPad, GoodReader for iPad, Cloud Reader, Plant vs. Zombie HD, Osmos for iPad, Atomic Browser.
 
Usual standard Apple apps like Safari, Mail, Photos, Youtube, then-
Kindle,
Good Reader/iAnnotate,
Dropbox
VLC player
Skyfire/Atomic browser
Evernote
Pages

I was surprised to see so many votes for pages. I just don't see the iPad and Word Processor really being a match. The iPad just feels more suited to a good text editor. Still trying to decide between Notetaker HD, Notes Plus, Penultimate and Audiotorium.

True, though it's good when I want to draft something and only have the iPad at hand.
 
Pulp is one of my favorite and most used app for catching up on the news. It's the best looking one imo and I especially love the Mac like feel quality of the app. Definitely recommended as a truly amazing app.
 
I'm a graphic design student who, at this point in time, can't afford a Mac laptop or desktop. I rely on the iPad app Inkpad for vector work. It's pretty much as powerful as Illustrator...minus all the bells and whistles. But down to the bone essentials...you could use it to do a major portion of design work if not entire projects.
Brushes, Sketchbook Pro, and Art Studio are other art programs. I spent about $40 all in all to put tons of art apps on it. I've used all of them in school projects.
And now a smiley for effect...
 
I would get an iPad just to run Touch OSC, for VJ'ing, Customizable touchscreen controls of live visuals! Thinking of selling my old powerbook and 2G iphone to get one...
 
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