Does it exist in tourneys, or just ring games?

And at what stakes can I expect collusion?

Note I'm not trying to find excuses for my poor play, just wondering what you guys experiences are.

You can expect minor collusion at every stake. E.g players on same table discussing cards through MSN or by phone. However as Stevo has previously said 2 idiots are not much better than 1 idiot and people good enough to collude properly can win without collusion.

Unless your playing $10/$20 upwards I wouldn't worry about it at all.