I use Codesmith tools, never touched subsonic, so guess which I'm posting about. :)
Codesmith tools is a template engine. You make a template with some inputs and it outputs the template with the variables in place.
I use it EXTENSIVELY in my module development. I have it generate everything I need to just start coding on a module. Including my test project for the module.
It's used to speed up generating identical code. Aside from my own module templates, I use netTiers for my DAL and that uses codesmith [the reason I bought codesmith originally]. It generates a DAL for my entire database without me writing a single line of code. Some of my projects tag upwards of 40 tables, so doing this manually, or writing my own [which I did for a while] isn't nearly as nice or robust as nettiers is.
There are other DAL projects that utilize nettiers to generate the DAL for a given database.
Codesmith also ties into Visual Studio, so it's possible to write small 'snippet' templates that you can run inside visual studio and have the code generated. I'm going to be looking into this soon because of the duplicative nature of a lot of code I do in my code that I can't automate at generation time.
Subsonic looks like it's 'equivalent' to netTiers. Generates a DAL. I like codesmith because it can do more than just a DAL.
hth