I can't say anything about other sites, but on UB in the $50 and $100 PLO games the river is often misplayed, especially by "straightforward" players. Stealing pots on the river and getting value from your medium hands can make the difference between a small and a huge profit for the session. Two hands to illustrate how to take advantage of people who are not thinking properly on the last street:

Hand 1) I'm in LP with [9c][Qd][Kc]. One limper, I limp, Button raises. Limper calls, I call. Flop: [Tc][Jh].
Button is straightforward, not too imaginative, and has $70 in his stack. I have him covered.
We check to the button, who bets pot. Limper folds, I call. Even though I have a huge draw, I don't want to raise: first, my opponent will fold a wide range of hands to a raise here, and I want to get his stack when the non-obvious draws hit. Second, if he does have a good hand he'll reraise and I won't be able to get away cheaply if the turn pairs the board.

Turn: .
This didn't help anyone. I think about betting out here, but decide to check. Opponent bets $30 into the $40 pot. He doesn't have a set, and is probably betting a two-pair type of hand. I call.

River: .
I have no chance of winning a showdown, and put him in for $20. The pot is $105. Opponent thinks for a while and folds showing KKxx.

My play on each street is debatable, but on the river there is only one move, especially against an opponent who does not think in terms of being ahead a certain percentage of times. I played like I had a draw all along, and he should have figured that I'd try to shoot my way out.

Hand 2) I have AJT3 on the button, and limp after 3 other limpers.
Flop comes [Jh][3d].
First limper bets $2, I am the only caller. I didn't raise here since I knew he was a calling station who wouldn't fold any draw, so I better wait for a safe turn card.

Turn: . Fairly safe. Limper bets $2 again and I raise pot. Limper calls.

River: . Limper checks. Pot is $38, opponent has $40 left. Opponent checks. Normally I'd check behind, but against this opponent I think this opponent will call a value bet with almost anything. I bet $15. Opponent raises allin. Not what I hoped for. However, I know that he almost always bets out with a made hand, so I call (also important for later play), and my sleazy two pair wins.

Conclusion from 1): don't give up on the river if you get good odds to try a steal. Try to have some money left on the river in case you miss your draw instead of raising allin on the turn. This is the corollary to the "save the last bet"-doctrine: not only can you save your bet in case the river pairs the board, but you can also use it to steal the pot when a scare card hits. Conversely, if your opponent has been drawing and all of a sudden bets the river, call if you get good odds and if you have anything at all.

Conclusion from 2): if your opponent makes an incomprehensible move on the river and is normally straightforward, call liberally. It will also prevent opponents from bluffing you on future rivers, since they see you make a dubious call with a weak hand.

Pieter