Online Speed Tells

When an opponent does a fast check it usually means that they are holding a mediocre hand unless he always uses the pre-select buttons.   You should be worried if a player re-raises after checking immediately because he could have been trying to trap you into betting out your hand.
    Online poker players who pause and then check their hand usually have a drawing hand.  These type of players want a free card from you.  Knowing this online tell, don't let the online poker players get their free card from you.
    Online poker players who take a while to place their bet usually have a great hand or they think that you're bluffing and want you to know it.
    Another online tell is when a player raises quickly on the river.  Usually this type of behaviour means they have a good hand or they are in a desperate attempt to bluff you out.  Once you figure out how the player plays the river you should be able to know when to raise or fold.

Online Chat Tells

Online poker players can pick up a lot of tells from the chat box.  They can pick up the type of poker player they are playing against and what kind of mood they are in.
    It can be easy to spot an online poker player on tilt.  If the online poker player suddenly become sarcastic or mean to the other players, he is usually going on tilt. an online poker player that just can't stop giving free advice to everyone usually is a poker player who has just recently made their first final table.  These players are probably not that experienced and should be easy competition.

Avoid Giving Online Tells

Most online tells that online poker players give are due to their betting speed and their chat.  Players should try to bet at the same speed to avoid giving online tells.