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.

T4 Editors 

A comparison of various T4 editors in the market.