You can use text templates to author your own code generators with intellisense & syntax-highlighting in Visual Studio.  This naturally has powerful applications for tool vendors looking to output code from non-textual representations of a domain such as diagrams, xml or other data sources.

Tangible has created a T4 editor to help you create T4 Text Templates.  The Tangible T4 Editor provides statement completion & intelisense for the T4 code sections.  Their editor also integrates with the error list to report errors on mal-formatted T4 code blocks.  A rich template gallery allows you to browse many existing templates.

The Tangible T4 Editor is up on the Visual Studio Gallery here: http://visualstudiogallery.msdn.microsoft.com/en-us/050a2994-cd28-4d1f-a185-9814e3389f4c.