Which mobile clients allow payments using Near Field Communications (NFC)? Which Bitcoin apps (or mobile-enabled websites) support the ability to pay using Near Field Communications (NFC)?
Andreas Schildbach's open-source Bitcoin wallet supports this feature. With the "Request bitcoins" screen open, tap another device to send it the bitcoin: URI to send to (that opens the "Send bitcoins" screen on the other device). You can also scan an NFC tag containing a bitcoin: URI to open the "Send bitcoins" screen with the fields filled in.