Mountain Goat software makes a great online planning poker application at PlanningPoker.com (funny enough). After registering for free you can create games and invite your team members to participate and cast votes for User Stories.
We used this for our distributed estimation sessions but entering all the stories was bit laborious. There is a great option when you create the game to import stories in a comma delimited format. When you are using Team Foundation Server you can create a query to export selected fields for your User Stories and add them all at once while creating a new game.
Create a TFS query with the fields you want to show in the planning poker game like the one below.
Open the query in Excel.
Then save the spreadsheet as a CSV.
Then open the file in Notepad.
Copy the column header rows and the rows with the User Story data. When you create a new game on PlanningPoker.com there is a field where you can paste the rows to import them.