Ask HN: "mandatory" apps for my new iPad mini

Hey all,

Just got an iPad mini and I'm really enjoying it. GoodReader, Kindle, Instapaper, and Flipboard means I have lots of high-quality reading material from academia, blogs, and the bookstore. The game selection is also great--King of Dragon Pass and Final Fantasy Tactics are the type of games I've been looking for on other platforms.

What am I missing, though? I'd like to hear about any apps that you get a lot of mileage from. "Mandatory" in the title is intentionally ambiguous--I'd like to hear about apps that you can't live without.