I would be lost without Evernote.
aWallet is a pretty good password generation and management tool.
Field Trip is indispensable
for when you're visiting a new city.
Hotel Tonight works well for when you're trying to book a hotel at the last minute.
Shortyz is a great free crossword puzzle app.
Also, Taxi Magic is great. You can use it to book a taxi, and it works in quite a few cities. So, so handy.