There are many established vendors of accounts payable and accounts receivable software and services. If you're ready to start shopping, the following descriptions of programs should provide some basis for your initial inquiries.
Bookkeeper
Avanquest's all-in-one
Cougar Mountain
Cougar Mountain offers accounting solutions with AR/AP functionality for small to midsize businesses.
CheckMark Software Inc.
CheckMark offers multiledger small business accounting with AR/AP. There are versions for both Windows and Mac.
Intuit QuickBooks
QuickBooks is a popular small to midsize business accounting application. It comes in several different editions, depending on your business needs, but all include AR/AP.
Microsoft Dynamics
Microsoft Dynamics offers several different accounting solutions aimed primarily at midsize companies. All solutions feature both AR and AP capabilities.
Microsoft Office Small Business Accounting
This is the software giant's small business solution, complete with AR/AP and more.
NetSuite
NetSuite offers an online-only integrated business management solution, including AR and AP. It is aimed primarily at midsize companies. You access the system using your Web browser.
Sage
Sage, formerly known as Best Software, owns a number of prominent brands of accounting solutions including Peachtree, MAS, and ACCPAC. All offer AR/AP capabilities, along with much more. This is for small to midsize companies.