Lots of people use the iPad for different things, from video to news, games, or as a laptop replacement device.
The indispensable thing I use my iPad for, besides class lecture outlines (PlainText) and keeping attendance, is reading stuff from the internet. And this is where Reeder and Instapaper are indispensable.
Used to be, I left a browser window open with lots of tabs, which were all the things I came across during the day that I wanted to read. Stuff would hang around there, and I would either go through them to close the window, or forget about the page, and then quit Safari or restart, and then run into, what do I do with these pages now? If Safari crashed, I would rebuild the links through the history, but it was a real pain.
With the Instapaper link on my laptop browser, I can send stuff to my Instapaper account whenever I like. I can even do this from the iPad and the iPhone. Then, when I launch Instapaper, either as a webpage or in the iOS application, there it all is, beautifully preserved and ready, and sans the ads and extraneous links--just glorious information. Links can be sent from anything on the internet, such as facebook, email, and twitter.
Reeder is the great RSS experience, where subscriptions to websites can be quickly scanned and sent to Instapaper with a simple swipe. I almost never use an RSS reader on the Mac, since Reeder on the iPad is so terrific.
So my late evening ritual? Time with Reeder and Instapaper on the iPad, where I get caught up on news, technology, stocks, and so on. They are the applications I use daily, and which make the iPad such a great experience.