The following 3 websites are recommended:
This is regarded by some as the best backgammon software program in the world. It is available for playing on a PC and in a portable version, by downloading the App.
Two other recommended sites for portable devices are:
Most other backgammon playing sites, by comparison, are feeble.
Playing for money
Many online backgammon websites are not looked upon as safe.
Generally, it is best to tread very carefully, using only websites recommended by backgammon players known to you. Also, never play for large sums of money because you could be playing against several players or even against a player using a software system like XG.