For what purposes have YOU used T4?

I've used the T4 Templates within the sharp-architecture to generate everything from models to controllers to basic views.

Definitely worth checking out, even if you just want to see some advanced examples of T4 templates in action

I am not a big fan of the stringy-ness of app.config/web.config, so I use T4 to read those files and make an AppSettings/WebSettings class that wraps the connection strings and key/values in a real class. This means that, as long as I always use AppSettings.SomeValue to reference my app.config, I get compile time checking, which is really nice.