You have many options for accounting software as stated previously.
It all depends on the number of tenants, how automated you want the system to be, and most important, your time commitment.
Quicken Property Manager, was design for owners with less than 10 units, very limiting.
Excel and Word are excellent programs that YOU would have to design templates to manage your properties. You probably have the software on your computer right now and depending on you expertise, this may be just fine.
Out of the box property management programs will give you the most flexibility and simplify your life too. Prices range from tens of thousands of dollars down to a few hundred. Yardi, Timberline, MRI (Intuit) are enterprise class solutions with prices to match as well. In the more reasonable price category are names like Tenant Pro, Tenant File and Rent Manager. All have their strengths and weaknesses. For me Rent Manager XP won because they have a long history, great technical support and the application continues to grow with me. For most on this forum it would be important to know that Rent Manager was initially designed as a mobile home park management application, an added bonus for us all.
Check out each company