Microsoft Dynamics NAV

And because it is from Microsoft, it easily works with the systems that your company already has implemented.
By automating and streamlining financial, customer relationship, and supply chain processes, Microsoft Dynamics NAV brings together people, processes, and technologies, increasing the productivity and effectiveness of your business.
Dynamics NAV – Learn More
There are a number of resources available online that you can use to find out more about NAV, developing for NAV and how to use NAV.
If you are learning about NAV and its features a good place to start is the main Microsoft NAV page (see: http://www.microsoft.com/dynamics/en/us/products/nav-overview.aspx).
The first stop for new NAV users or new customers is Customer Source where you can access self-support functionality and search for help. (See: http://www.microsoft.com/dynamics/en/us/customersource.aspx)
Development:
Microsoft Dynamics NAV MSDN Home: http://msdn.microsoft.com/en-us/dynamics/nav/default.aspx
Microsoft Dynamics NAV 2009 Developer Documentation: http://msdn.microsoft.com/en-us/library/aa306436.aspx