With the in-turn buttons you can see the most obvious tell that shows up in online poker. If the flashing light representing a fellow player acts straight away, it's more than likely this player has clicked thier act in-turn box. Sometimes it's very easy to workout when a player is holding a strong hand as a fast "check" can often be very revealing also. So if you are first to act in a hand, and for some reason take a moment before checking, and your three opponents immediately check behind, this can be a tell that they are not holding good cards.
Another all to common situation that arises, the first player pauses for a moment and then finally checks. You have the "bet/raise in turn" button checked, so your bet quickly is placed, but then instantly the player next to you raises. He must of had the bet/raise button checked also, and he wasn't scared of you or the first player or what you did!!! From this you can determine he has a strong hand and unless you've got a big pocket pair you'd better fold.
Other online tells can be found from how slow a player acts on their hand. Players who are consistently very slow either have a slow internet connection or are not concentrating, either because they are playing two games and are not experienced enough or because they are distracted in some way ie busy doing something else on the pc. Either way, if suddenly this person plays a hand fast and aggressively... then again theres a good chance he's holding a good hand and is chomping at the bit to get his chips in the pot .
The long pause or stall is a common tell among average poker players . When the last card (the river) comes onto the board a player will pause for sometime as if having to really think about there hand, beware of this as usuall this player is holding a monster and has been slow playing you in, also some players try to use this tactic as a double bluff, by pausing to think with no hand and then acting strongly.
The last group of online poker tells I'm going to tell you about can be found in your opponent's betting patterns. Betting patterns reveal quite alot in online poker especially with the volume of hands you can play in just 1 hour. In particular, people playing multiple tables make a huge amount of bets across the many tables they are playing. With playing such a large amount of games comes repitition(we are only human). Watch your fellow players long enough and you should have some good clues to what they are holding, and especially how interested they are in the current pot.