There are no easy tells online, primarily because you can't see physical mannerisms.
I'm guilty of one big on-line tell, but it actually works in my favor. Here's mine...
Everytime I flop the nuts, or DAMN close to the nuts (ie hit my set), and a player makes a HUGE bet,.... I pause for several seconds.
Now normally I'm one of the fastest players at the table, so the pause is very noticeable. Obviously my intention is to make them think I've got a marginal hand and their huge bet made me stop and think.
After a good 10 seconds... I call. (assuming I don't need to push them out of the hand... ie flush draw). The slow call works like a charm! On the turn the player almost always makes an even bigger bet convinced I'm on a marginal hand. The trap is sprung!!!
I've induced countless monster turn bets slowww calling, that NEVER would have happened had I insta-called (or even raised) their big flop bet.