Apps and services for splitting and tracking bills and expenses between friends

I'd like to start using a bill/expense splitting service, like splitwise, to request payment and track shared expenses between friends and housemates.

Would really appreciate opinions on what services/apps I should consider (so far I have only really looked at splitwise)

In my mind, a good option would

a) be cross platform - at least web, iOS, and Android (Windows Phone would be a bonus)

and

b) have pay through paypal (or some other electronic payment system) integrated into the app.

Therefore a comment on whether these features are present would be really helpful.