Uncertainty in the Product Backlog is a big risk for the schedule of a Scrum project. The problem is that the full scope of the release can be quite hard to estimate because the requirements are not well-know early on. Confounding this problem is that frequently the release date is a hard deadline.
The first thing to say is that the term “Planning Poker®” is a registered trademark of Mountain Goat Software, the Mike Cohn company. This is why this agile planning practice could be sometimes names differently: scrum poker, agile poker, etc. This article presents a list of free and open source planning poker tools and plugins that are also directly available as online tools.