A few of mine......
gymspot is a good one, same kind of feel as here nice bunch of parents and gymnasts. They also have a forum for gymnasts only, which I think is nice for the younger crowd. Parents forum is for the over 18 crowd, but not a place for adult content.
In Canada gymbrook is interesting when anyone posts, very quiet in the non comp season, lots of gymnasts post. Only problem is you can post as a guest, so it is hard to work out who is who and who is replying to what.
GGMB is a riot, but it is impossible to join now as they have strict mebership guidelines, I was in before the rules changed. It is definitely a place for the over 18's and not for the prudish type. They are bold and brash, but very funny and you cannot take yourself too seriously there.
WWGYM is a newish board formed from some old GGMB members. Both GGMB and WWGYM are ary into the international gymnastics scene and not at all into low level kids gym. It is nice to have a few to choose from.