I've written a post about N-acetylcysteine before (http://garthkroeker.blogspot.ca/2009/09/n-acetylcysteine-for-treatment-of.html), which suggested that it could be useful in treating compulsive behaviour disorders such as skin-picking.
A recent 2012 study by Afshar et al. has shown that NAC is useful for treating obsessive-compulsive disorder (OCD). Here's the reference: http://www.ncbi.nlm.nih.gov/pubmed/23131885
In this study, 48 patients with OCD who had not responded to an SSRI were given NAC up to 2400 mg/day or placebo, in addition to a continued dose of the same SSRI, for 12 weeks.
The NAC group had about a 40% reduction in YBOCS score (a quantitative measure of OCD symptoms) after 12 weeks, compared to a 20% reduction in the placebo group. This is a good, clinically relevant symptom change especially for a treatment-resistant group.
Mild gastrointestinal complaints were more common in the NAC group, but there was not a big difference in drop-out rates between placebo and NAC.
NAC works as as a glutamate-modulating agent, with possible anti-inflammatory effects. It is metabolized to the amino acid cystine after entering the brain.
So it appears that NAC could be a simple, low-risk, effective adjunct, or even a primary treatment modality, for obsessive-compulsive disorder.
I would be curious to see more research looking at NAC for other anxiety disorders, or for ruminative depression.