Planningpoker.com is one of the original online tools, and one of the most consistently supported. It offers great integration with common platforms like JIRA as well, and a lot of large and small companies use it. Fair to say, it is the standard by which others are judged if not the best.
PlanITPoker.com is probably the main competitor to Planning Poker and is an excellent tool in its own right. It basically offers the same features and is similar enough that it is really a matter of preference as to which one you use.
This one is simple, elegant and the easiest to start with and use. Almost no inputs are required compared to the other two, so you can start a session very quickly - no login required!