Simulating mixed models

I've been sitting on this post - no longer sure why I have it bookmarked nor why glmm models are being simulated using a gamma or negative binomial distribution. At some point I must have been wanting to do this but in the context of a multi-level model. I'm not sure how this has any relevance any more which reminds me that the adage "Do it now" really needs to be applied in my case.