One of the coolest features in EntLib V3 is the environmental overrides feature.  This helps you manage differences in configuration between dev, test, production, et al.

Managing differences between configuration files can be done in the same view you would edit configuration settings for a particular “Configuration Node”. For each environment configured in the editor you will have the possibility to either “override” the settings for a particular environment, or keep the settings for this environment in sync with the original. Configuring the environment can be done from within the configuration-treeview, adding new environment nodes underneath the “Environments” node.

This is super cool because it's been a real PITA to handle this before now.

Download EntLib V3 here.