As sanuk says, either Quickbooks or Sage (formerly Simply Accounting). I wouldn't bother with anything else for a small business.
I haven't looked at the pricing recently, but last time I looked the pricing for Sage was better than Quickbooks. Depends on what you need it to do.
edit... what industry / nature of the business? Requirements? Do you need it to do payroll and generate T4s?
That'll help people give a better recommendation.
For really small business (no employees, non-incorporated, etc) then I think Quicken will also work with the added benefit of integrating into QuickTax. I don't recall if Quickbooks will integrate easily into QuickTax or not.
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