In addition to the exbet.io mentioned by TimeTeller, the following websites have been (or are still being) promoted on the forum:

Fairlay - prediction market/betting exchange
Yebsite: https://fairlay.com/
ANN Thread: https://bitcointalk.org/index.php?topic=673475.0

DEGENS.com - The Blockchain Sports Betting Exchange
Website: https://degens.com/
ANN Thread: https://bitcointalk.org/index.php?topic=5222499.0


This is not a recommendation, since I have no personal experience with any of these platforms. Everyone should do their own research and due diligence.