Devart T4 Editor is described as 'powerful Visual Studio add-in for editing T4 templates with syntax highlighting, intellisense, code outlining, and all features of a first-class text editor add-in for Visual Studio' and is an app. There are three alternatives to Devart T4 Editor for Windows,.NET Framework and Microsoft Visual Studio. The best alternative is Codesmith. It's not free, so if you're looking for a free alternative, you could try MyGeneration or T4 Awesome.
MyGeneration is an extremely flexible template based code generator written in Microsoft.NET. MyGeneration is great at generating code for ORM architectures. The meta-data from your database is made available to the templates through the MyMeta API.