Use All The Features
When you play online poker, be sure to use all the features of the site on which you are playing.
I've been playing the 6-max NL tables on FTP almost exclusively lately.
The first thing I do when I sit is to do a player search on every player sitting at my table. (In the lobby, click the 'Requests' menu item and then select 'Find a Player...' and type in the player name, or copy and paste it from the chat window.)
I then put a blue mark on every player that's playing more than 3 or 4 tables at the same time. It seems there's always at least one playing 6-8 tables no matter when I play.
Those players are playing by the book. They only enter pots with pre-flop raises, they always continuation bet, and they only call a check-raise/re-raise if they have a monster. Call a lot of their raises in any position, and play back at them often. Don't get over committed because they will go to war with an overpair or usually even top pair, but they'll give up basically ANYTHING else.
I suppose there are other ways to take advantage of these multi-tablers, like stalling when it is your turn so they might get distracted by action at another table. I don't really do anything special except assume that they aren't going to get any trickier than a continuation bet.
Of course I'm not playing very high levels, and you might find less people playing tons of tables at the $2/$4 NL games, but at the stakes I play, it's a lot of fun. I'm still not really comfortable leaving a lot of money on FTP, so I stick to the low $25-$50 stakes games.
It's not like I have anything against people that multi-table, hell I envy them. I simply can't do it. But I'm going to take advantage of them if I can. It's the nature of the game.