Many customers want to integrate back office information into salesforce.com, including data from standard ERP or accounting packages or legacy, home grown systems.

The most common Salesforce integration scenarios include:

1. Customer Account Master Integration: For example, when a new account is added in Salesforce, you may want to either update the Customer Account master record in another system, or vise versa. If Salesforce is the master database, you may want to move the data from other systems into Salesforce.com. This ensures a high level of data integrity.
2. Invoice and Order History: For example, when an opportunity closes you might want to set up the account in your accounting package and kick off an invoice. The revers might be true as well. You might want to make your sales rep aware of the fact that a customer has an overdue balance without having them log into your accounting application. In addition, you may want to store order history data in Salesforce so that the Sales Rep has a single view into all the customer information.

Salesforce.com has a number of partners on the AppExchange who have built connectors for the major accounting packages, as well as provide solutions of integration to back office legacy systems. Here are a few:

QuickBooks
If you have QuickBooks try IntraWeave. If you have QuickBooks Online AccountDynamics has a great solution.

Microsoft Dynamic, Formerly Known as GreatPlains
If you use GreatPlains, Scribe Software has an AppExchange application to integrate with Salesforce.

Peachtree, Mass90
If you have a mid-tier accounting package such as Peachtree or Mass90, Pervasive has a great solutions.

Oracle Financial, SAP
For integration with Oracle Financial, SAP, or other enterprise products, Above All Software, Castiron, Informatica, and Pervasive all have solutions. For more information on these solutions, as well as 30 other solutions by additional partners, visit the Integration Category on the AppExchange. 

In addition, some customers have also expressed interest in on-demand account packages. Intacct, an AppExchange Partner offers a hosted Financial, Order, and Expense Management that is fully integrated to Salesforce.com.