Do you mean for PTSD or are you just curious what other sorts of support groups are out there?
I belong to a number of other websites but the PTSD section is just one of many forums on the site so you don't get the diversity of subjects and the amount of support that you do here. I've found that those sites are dominated by a few people and if you don't agree with them, then you're on the outs. They like to have things their way as a way of keeping themselves stuck, I think? I usually go to those sorts of sites not for the PTSD support, but to visit the other forums.
In terms of PTSD support, I think this is the best site. Of course there are other sites out there that can help, such as ones for childhood abuse, etc, that relate to PTSD, but again, I've found the tone to be a bit whiny and the people aren't so interested in finding a way to heal as they are just looking for a place to complain. (That is, they don't like the hard hitting constructive criticism as you need to wear your kid gloves there....and obviously its frustrating for me to not tell people what I really think....Sometimes the truth hurts, but they rather be coddled.) Honestly, I haven't been to that childhood abuse site in many years now because I never got any real support there other than things like (((HUGS))) and "it'll be ok...".
