The mathematical modeling SE proposal (http://area51.stackexchange.com/proposals/57490/mathematical-modeling) is closer to becoming a real site, but it needs more work. This site is for questions that are about models for real life phenomena. For instance, I was at a math talk yesterday where they described a PDE modelling bacteria with flagella, and they described how the model gave incorrect results until they ellipsoids instead of spheres and incorporated random 'tumbling'. This site is for people to ask questions about such models.
Any new site requires 60 followers (there are now 80+) and 40 'sample questions'. These are not meant to be answered; everyone who visits the site adds up to 5 questions (each <150 characters, the size of a tweet) and others vote on those they like.
Because many of the followers (including me) are theoretical mathematicians, we have trouble writing expert-level or even graduate-level questions.
If you can think of an interesting question on mathematical models (versions of TQFT, improvements on mean field theory, Einstein's treatment of viscosity for spheres) of 150 characters or less, please follow this link, scroll to the bottom, and ask your questions: http://area51.stackexchange.com/proposals/57490/mathematical-modeling
Questions of overlap with other sites are discussed on this site as well. This proposal has support from mathoverflow, mathSE and scicomp.