I probably use Instapaper more than any other app. The simple, readable design makes for a really nice experience. The New York Times app aims for a similar simplicity, though I find the ads kind of intrusive. In both cases, being able to read while offline (meaning, on the subway) is a major plus.
I’m using Birdfeed as my twitter client right now, though I think I like Tweetie just as well. Birdhouse is great for writing down things you think you want to tweet, only to realize later it’s not such a good idea.
Satchel hooks me up to my Backpack account, which I use to keep track of all the various projects I have going on at any given time. It also helpfully works while offline, so that I can dash off notes while commuting. I don’t love the UI, and it can be slow to synch up, but I’m confident these things will improve in time, and I’m too addicted to my Backpack account to give up on it.
The Typography Manual is a beautiful little reference manual; just being able to rapidly look up the HTML character entity for an em-dash makes it worth every penny. And it’s a great model for how “book” content can make for a usable, readable application. Next to it is What The font? which allows you to take a picture of a font in the wild and identify what it is. It doesn’t always work, but when it does it’s like magic.
Ego allows me to peek into the stats on A Working Library from time to time to see how many people are reading. Helpful for any writer who needs regular assurances that she should keep writing.