Every small business needs to keep accurate books, separate from personal finances. Two popular choices are Microsoft Office Accounting or Intuit QuickBooks. I've used QuickBooks in the past as the treasurer of the 301 Gough HOA and it definitely is the market leader, so QuickBooks won despite my heavy Microsoft leanings.
I decided to go with the QuickBooks Pro version, which is the intermediate package in their business product offering. A little Googling lead me to a software discount site, that saved 20% off the list price of $199.95. Gotta love Google!