Cloud book keeping software for property agents?

Does anybody know of any book keeping software available on the "cloud" suitable for property managing agents who might want to keep sets of books for each property under management for service charges purposes but have ready access to all sets of books simultaneously for , for example, credit control purposes? Somebody asked me this yesterday who is currently a small start-up business (with a small start-up budget) but who wants something dependable and very "scaleable".