Which mobile apps allow me to redeem a private key from a QR code? Which mobile apps will let me scan a QR code of a private key (such as the one created by BitAddress) and then spend the funds from that coin?

Please specify Android, iPhone (iOS), etc.
Mt. Gox mobile, available from Google Play and on jailbroken iPhones using Cydia allows you to redeem private key QR codes into your Mt. Gox account.

As a bonus, if any payments are made to that address in the future, Mt. Gox will automatically sweep those funds as well.