Products and Extensions in Codeplex
A few useful ones:
T4 Toolbox by Oleg Sych
T4 Toolbox is a set of ready-to-use code generators and T4 extensions for Visual Studio. Quickly build powerful C# or VB templates that generate multiple output files or simply use existing templates for LINQ to SQL, Entity Framework and more.
A comparison of various T4 editors in the market.