I had an iPod Touch for a while so I could explore the platform before deciding to buy the iPad or an iPhone. I found the iPod Touch quite useful, but nothing like the iPad (despite my original impressions, it's not just an iPod Touch on steroids).
Now that I've used the iPad for a while, I thought I might share what I find to be the applications I use the most.
Mail - Well, doh! One of the things I really appreciate about the iPad though is how easy it is for me to create connections to all of the various email account I need to access and how user-friendly the interface is to use for all of them. So much so that even while I'm in my office and working on my Windows workstation, my co-workers will find me answering their emails on the iPad. You may need to make sure you go into the Info section of your device in iTunes and sync to the address book of some of those email systems though if you want to be able to create new emails to them from the iPad. Fortunately, the iPad keeps the various sources of the contact information straight. When I was attempting to sync to various email systems from my Windows Mobile device, it ended up syncing the various sources with each other as well.
Calendar - Another doh! But what I'm really surprised by is how well the iPad will sync with a number of different calendars and yet not mingle the events with the host calendars. It means I can sync with both my personal calendar on Yahoo and my work calendar in Exchange and show both sets of events in the iPad calendar. There is a color indicator on the events indicating which calendar it is from. When I was using a Windows Mobile device to sync with my calendars it would end up adding all my personal events to my work calendar and all my work events to my personal calendar.
TextPlus (ad-free) - I can't make calls on the iPad (or at least I haven't tried yet, I can't imagine holding it up to my ear <grin>), but I can and do text message from it.
Newstap - Folks that know me from the Sybase forums know I spend a great deal of time in those NNTP newsgroups. Newstap is a great way to access them from the iPad
Pages - Allows me to work with Microsoft Word documents on the iPad. You transfer the files to the device and back through iTunes, it a spot on the bottom of the page under Apps when you're viewing the device. It's rather hidden. Note that as far as I know, none of the other iWorks apps (e.g., Slides, Numbers) handle Microsoft Office docs without going through a conversion via the full version of iWorks, so I can't recommend the others.
[caption id="" align="alignnone" width="150" caption="Where Apple hid the file sharing option on the Windows version of iTunes"][/caption]
Kindle - So I can read my ebooks from Amazon without having to use a Kindle reader.
Print n Share - Allows me to print documents directly from the iPad.
Weather HD - There are a lot simpler weather apps out there, and others that do a lot more. I'm just stunned by the images used in this particular app.
TapTrace HD - One of my biggest concerns with the iPad is that I'll put it down somewhere and forget to pick it back up. It's not like the iPhone/iPod Touch that I can just slip into one of my pockets. At least not until I get some pants with some really, really bigger pockets. To help ease my mind a bit, I picked up TapTrace and placed it on the bottom of the home screen so that it appears on each home screen page. If you lose the iPad, you send a message to it from a regular PC through the vendor's web site, and a message icon appears on the application icon on the iPad. If someone finds the iPad and opens the app, the location of the iPad is transmitted back to the vendor's web site. The application also gives the person who found the device a message indicating how to contact you, and allows them to send a message directly to you from the app. Won't help if the person who finds it doesn't intend to give it back, but it would help honest people get in touch with you.
iTap RDP - A great Remote Desktop tool for accessing your home or office PC. It uses multi-touch gestures to implement mouse actions and to launch the keyboard. It also allows zooming.
Plants verus Zomies HD - When you need to work off a little frustration, take out a few zombies. I love the PC version of his game, which has quite a number of extra features (e.g., Zen Garden). The HD (iPad) version has quite a few more features than the stripped down version for the iPhone/iPod Touch though.
Zombie Smash - If killing zombies with plants isn't enough for you, Zombie Smash allows you to kill them with a wide variety of weapons, including just your fingers.
A few other tips for you:
Creating safari bookmarks on the home screen - Rather than lauching Safari and then launching your favorite website from a bookmark, save the bookmark directly to the home screen. It's a great way of making some of your most frequently used web sites appear to be native iPad apps.
Video conversion - The iPad supports syncing not only photos but videos to the device, but the videos have to be in an appropriate format (e.g., H.264). I found that the open source Handbrake to work well with most files, particularly those from my digital camera. However, most converters (including paid ones) seem to have problems with the 3GP files created by my mobile phone. I finally determined that the Cucusoft iPod Movie/Video Converter seemed to do the best job handling those.