#1. The donk bet. When a player bets into a raiser from the previous street. It's usually done by a player who doesn't understand that the raiser is representing a strong hand. This applies mostly to heads-up because in a multiway pot it is sometimes correct to bet out. This is how it goes:
Full Tilt 0.50/1 Hold'em (6 handed) Full-Tilt Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)
Preflop: Hero is UTG with Q, T.
Hero raises, 3 folds, BB calls.
Flop: (4.50 SB) 3, A, 5 (3 players)
BB bets, Hero raises, BB folds.
Final Pot: 3.25 BB
But sometimes it's a legitimate hand, but the player didn't choose to check/raise with it. It is still useful to know that players do this quite often with nothing or a draw.
#2. The double check/raise. First the player check/raises you on the flop, you call and improve on the turn. Then he checks the turn for some reason. You bet and he raises! This is usually a player who doesn't understand the reason to use the check/raise play. His mistake is not leading the turn. This is also often air or a draw because if he did have a strong hand he'd be worried that his check/raise would scare you into not betting the next round of betting.
Any other donk plays that you know that let you take pots away from fish? Or alternatively get away from hands you'd otherwise call with?