Lets say you used vanitygen or vanity pool and have key/address on a pc.
You are paranoid security aware and do not want this key to be in an email or passed on to a website to make a qr code.
Paste key into app listed above it will make a text QRcode. QR Droid can read it on android device you can copy and paste into wallet. Or as I said Mycelium Wallet can scan QRcode (or paste from clipboard).
I'd feel better using an open source qr generator, built from source, for any keys containing large balances..
http://sourceforge.net/projects/zint/