Standalone Windows application - Master for Allen Bradley, some Omron and Modbus PLCs. It is using libplctag library and C# Wrapper. Also included is the Mono version for Mac OS X (which should work fine in Windows as well as Linux Mono - see notes).
The benefit in my view is that you won't need to have library embed/extraction/dll interfacing logic in your solution, you can use what is provided there. I also usually update this wrapper within a few hours of a release of the core library by Kyle unless there are API changes.
If not, are there any features lacking in this wrapper that would prevent you from using it?