Collusion in Poker?! Never!?

Collusion is two or more players acting with a secret, common strategy. Some common forms of collusion are: soft play, that is, failing to bet or raise in a situation that would normally merit it because you don't want to cost your partner money; whipsawing, where partners raise and reraise each other to trap players in between; dumping, where a cheater will deliberately lose to a partner; and signalling, trading information between partners via signals of some sort.

Simple collusion in online poker is relatively easy and much more difficult to immediately spot if executed well. Cheaters can engage in telephone calls or instant messaging discussing their cards since nobody can see them. Sometimes one person can be using two or more computers and playing under different aliases. This gives him an advantage that's difficult to work against.
However, online poker cardrooms keep records of every hand played, and collusion can often be detected by finding any of several detectable patterns (such as folding good hands to a small bet as it is known that another player has a better hand).

Another concern in online poker is the use of bots. These are programs that play instead of a real human. Though their accuracy and their ability to actually win are in dispute, their use does violate the rules of the cardrooms so using them is by definition cheating.

In a poker tournament, when one player is all in and two other players are active in the pot, it is common for the two players with chips left to "check it down". Unless they explicitly communicate an agreement about checking it down, this is not collusion.

From : Wikipedia

I strongly agree with the last paragraph here. Unless you flop the nuts or at least a set, there is absolutely NO reason for you to bet at a pot where there is another player all in! It is strategically better for you to eliminate another player(or allow them to be eliminated) as the more players knocked out, the higher you advance in the finish $.