Online Accounting Systems

This is a good question! I do not know of any practical bookkeeping system besides double-entry accounting. Theoretically, transactions are inherently double-sided, as for one to give, another must take. This creates balance, an equation, if you will. It is the only way math will actually work when you're dealing with numbers that have relationships. So I cannot think of any other sort of accounting method, at least that would be widely used and practical.