As a rule of thumb, if it looks like it sells a lot of hardware, then it'll likely be a competitor. Most places are common sense, some a little grey such as Game.net. Things like the latter, they don't sell enough stuff to be classed as a competitor I belive, you're okay with posting games related stuff, but if you go posting hardware links for cheaper motherboards for example in the motherboard forum, then you'll get the post deleted.
It really isn't that hard, and it's not as easy as fully starring out terms. Say
******** was a competitor, if somebody were to post a link for it, it would become
www.********.net. If we have then a conversation about how good the gameplay of a game is, we lose the right to post that word, and makes thigs rather annoying. Same with many of the competitors you can probably think of.
If you are really not sure about if a certain site is going to be iffy, then find a mod or myself on MSN/E-Mail and ask, don't post it straight away. I'm online most evenings, as are a lot of dons, and can quickly say yay or nay to things.
It's not as simple as giving a list of okay sites, as there are hundreds, of them out there, and people would be scared to post minor obscure sites due to them not being on a said list.