After a few weeks of lots of hard work we were finally able to release a pretty stable release of our T4 Editor out to the community at http://www.t4editor.net.
It currently support VS 2005 and VS 2008 beta 2 and has most (but not all yet) of the bells and whistles you would expect from a nice and integrated editor in Visual Studio (meaning IntelliSense, syntax coloring, region collapsing, etc).
If you're into anything like DSL, GAX or Software Factories then this is a must have for you.
And if you're using any of the code generation tools out there and you've haven't heard of T4 yet, then it's probably time you give it a look...
You can go directly to the download page and give it a try, I would love to hear your feedback if you do so!