Earlier this year I did a series on Implementing Post Master Enterprise from Envisage Software.
This is one of my favourite addons for Microsoft Dynamics GP which I have implemented for quite a few customers, including two in the last month. The basic functionality is that it will run as a Windows service and automatically post batches which match the autodetection rules. Envisage also make regular updates to the product both to improve functionality and to fix any issues which arise.
While upgrading some products is complicated, upgrading Post Master is nice and straightforward. All you need to do, is uninstall the previous version and install the new following the installation for a new implementation and updating the serviec to auto start and logon with the correct account.