OK so this is a very personal view on the 'must have' apps for the iPad, but these are the ones I really wouldn't want to be without anymore. So, in no particular order ....
iBooks - I know, I know, it's the one pushed and recommended all over the place, but hey - it works! I love the library shelf interface and find it really easy to use. Adding the books through iTunes is easy enough and it reads PDFs pretty well. I use this a lot.
SoundNote - Now, to be honest I've not really used this in serious anger but the trials I've done with it have worked superbly. It records audio and relates it to notes as you make them, so if you scribble a brief note and think "what the hell does that mean" a quick tap and you can listen to what was being said at the time! Brilliant.
Pages - In a previous post I've described how I used this in a cramped airline seat. I find it really quick & easy to use and the keyboard suits my BFFs (that's Big Fat Fingers) so I can type with reasonable speed and accuracy pretty well anywhere (yes, I mean anywhere, but we won't go down that discussion again!). And it transfers to Word pretty seamlessly. Nice
Livedrive - OK, this needs to be setup on your home based machine (it's a cloud backup for those who don't know it - good value as well) and I didn't really think much about the link to the iPad, but it's actually a very good way of syncing files into the cloud and being able to access them from any machine, including your iPad. Upload time is painfully slow (~4 months for a full backup of my images!!!) but once it's done you can just access them to view or download. Simplez!
Twitter - Now I was a real Hootsuite on desktop & iPhone fan, so when they upgraded the iPad app I thought 'great', but it doesn't really do it for me, don't know why. Instead I find the Twitter app is simple and elegant to use with those slidey window things so you can open pages within a page without leaving the app.
Portfolio for iPad - Did you read my earlier post where I said the iPad was a disappointment as you couldn't select and rate images? Well now you can with Portfolio for iPad. It's nicely presented (you can add an image to the front page for presentation & branding) and you can rate and filter images. It also has a nice slide show feature and has the ability to hide galleries so you can choose what you show. Uploading images is easy enough but if you use a Mac there’s an application to do it all offline (I don't so can't comment on this).
Photogene - For me this is THE app to use if you want to edit photos on the fly. It's easy to use and packed with features - the recent upgrade has made it even better. I don't actually do much editing while I'm out and about but this app has some very nice inbuilt frames so I export all my web/blog images to this app before re-importing them again to be published. Cool app.
Guardian Eyewitness - With a new image posted each day there's some stunning images posted here (and some mediocre ones as well, but that's taste isn't it?) and I just find it fascinating to see what other people do.
FlickStackr - One of the many Flickr apps around, I like this one as I can view the recent activity for all users and see what other people are doing - nosey bugger aren't I?!
PosePad - Now this caused a bit of an argument at the studio when I recommended it. I like it as you can import images into 'Pose Books' viewing them as thumbnails or individually with the ability to add notes. I have 'grabbed' images from all over the place and added them for inspiration or to show models/subjects what I'm trying to get them to do. I've also built a pose book for a shoot so I can sequence what I wanted to do. Some folks don't like the fact the thumbnails are square and the images distorted to fit - doesn't bother me though!
FlightCtrl HD and MOD Conflict HD - There has to be some games of course and these two have got me hooked in equal measure, though I'm equally bad at both of them! Good fun though.
So these are my top choices. I have many more apps of course, some are used now and then, others very seldom these days. But those I've mentioned above are the ones used, well, if not daily then very nearly so.
Feel free to comment and add your own 'must have' choices - after all it's all about personal tastes!