PayApp is an app currently exclusively for the iPhone, it registers transactions between people and remembers it! This is a simple tool to help you remember who you owe money.
You do not need to create any accounts, PayApp uses your Gmail or Facebook account by simply allowing it access to your email address you make it possible for other people to find you and add transactions between you and them.
Your email wont be used for anything more than finding the transactions concerning you and the other part of the transaction, the contact with the server is done through ssl to safely transport the data.
All your transaction history is saved both online and offline, there is an offline mode that you can activate when you are abroad, when you set the app in online mode again it will sync all the changes with the server.
If a transaction occurs in any other currency, you can let the app lookup the exchange rate and calculate the result in your own chosen currency
The app lets you split any amount between any number of people (including or excluding you) in the case when some one pays for a round of beers or similar.
There is no need for the other persons to have the app, you can create a "local" user for them, only visible to you, in the event that they later choose to get the app you can simply transfer the transactions to their real account.