A great link from Simple Talk going over five database modeling tools:
Simple Talk – Online Database Modelling
The tools range from free to a monthly charge, but all are way more affordable than the usual enterprise modeling tools.
I didn’t try them all but I did like the Vertabelo tool. Vertabelo supports SQL Server 2012 (along with other systems).
Vertabelo also has a great blog with articles on database modeling and design.