Continuing into my other most useful apps (but not ones I need every day) …
WordWeb is the best dictionary app I've found for offline use. It also has nifty tools for word puzzles such as crosswords. I think it was free.
it's more than possible to read Wikipedia in a regular browser, but Wikiamo is a purpose-built app which is better designed for a small screen, and has some other added benefits. But if you use an iPad, then I suggest Wikipanion has a more appropriate interface.
TreeID was an impulse purchase when walking through fields with the in-laws, and we wondered what a particular tree was. Having just bought the iPhone I muttered "I bet there's an app for that", and then discovered there was. It's not fully comprehensive, but quite fun all the same.
WhatTheFont is also definitely in the fun camp. Being a font geek sometimes you just need to know what a particular font is: with this app you take a picture of some text, type in the letters it represents, and then a minute later you have the best guesses from the website of the same name. Almost magic.
