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Boxcars, a program for playing backgammon online and offline, is coming soon to Steam
(store.steampowered.com)
bgammon.org is a free and open source online backgammon service without ads. Registration is also optional.
Join the community at https://bgammon.org/community
There are two engines available to play against, tabula and wildbg. tabula is rated beginner to intermediate level, and wildbg is the highest rated player on the server currently.