Commercial advertising is an obvious no. (especially with the bots we have been getting, they are all scams / fake products anyway). That's just an abuse of OPU to try to drive traffic for commercial purposes (it can also negatively affect our position in search results, if we have links on our site to scam websites, or websites that have no content and are just meant to try to boost search engine rank (doorway pages and the like)).
Links to personal forums/whatever where you aren't trying to make money... well I guess I don't really see a problem with this, so long as the idea isn't to divert traffic from OPU. I don't really see a problem with something insignificant like in a signature or whatever, but not posts specifically to promote said website. That being said I don't think posts where legitimate technical issues are being asked (good example being op2rules asking us to try to perform some security tests on his website) are really an issue at all. Basically so long as it's not simply a blatant advertisement for a website as opposed to having a question answered about it. I think there is a lot of grey area as to what defines "blatant advertising" here and will probably have to be handled on a case to case basis.
The only exceptions I can think of off the top of my head would be personal website posting/whatever in the wrong forum (general and test forum exist for these purposes), or OP2 forums of some sort (which falls into the "diverting traffic away from OPU" realm; if people want to encourage discussion about the game they might as well point people in our direction).