As someone has already noted, VLC is an open-source media player that is available for every major platform out there: WIndows, Mac, & Linux.
Everything I can find indicates that the versions since 2.0.1 should have Blu-ray support shipped with them. However, they also have a page that details libbluray and on that page they have this NB:
People interested in AACS technologies should have a look at libaacs.
People interested in BD+ technologies should have a look at libbdplus.
It appears that you may need to download one or both of those additional libraries and install them to allow VLC to play back commercial Blu-Ray discs, which is what's been under discussion here.