Google Wallet which was launched a few months ago is now available as a standalone APK installer for Android phones, if you have a NFC (Near Field Communication) compatible Android device then you can enjoy the Google Wallet service without rooting or any custom ROMs, just download and install the Wallet.apk from below and install as you normally install any third party apk on your Android phone.
Download Google Wallet Android APK
Wallet download link for Nexus S
Google Wallet Demo, How it works
Official XDA Thread
Here is the demo of Google Wallet apk running on Galaxy Nexus