It is not often we recommend a Microsoft product, but when it comes to editing and working with the OpenSim source code, the beta version of Microsoft Visual Studio Code came to the rescue.
Having an editor on OS X that is .NET aware made an improvement over UltraEdit, and it is very fast too. The side by side editing mode on a big screen comes handy, as is having access to full folder hierarchies on the left hand sidebar for quick navigation.
With mono installed one can also compile and debug the code right from the editor.
You can get the beta, which is also available in a 32-bit Linux version, and more information from here.