If you use both Mac and Windows and need a player that can play Blu-ray on both platforms then take a look at Macgo Windows Blu-ray (previously Blu-ray Player for Windows).
Macgo Windows Blu-ray recognizes Blu-ray devices and internal Blu-ray players instantly, meaning you can connect a Blu-ray player to your PC and watch Blu-ray discs. Not only that, but Macgo Windows Blu-ray supports almost every format out there so you can use it as your default player if you choose. Even better, for the first three months it's completely free to use, although a watermark is placed on the screen and some features will be deactivated.
To play protected Blu-ray movies however, you need an internet connection because Macgo Windows Blu-ray has to acquire a license to play the movie on your system. Macgo Windows Blu-ray is fairly basic. There are very few options other than to add subtitles and change the audio track. When you start it up for the first time, it will also prompt you to select which files you want to associate with it so it won't take over your media files unless you allow it to.
Macgo Windows Blu-ray feels a bit basic in design and you might experience some crashes and instability, but bear in mind this is the first release and the developers warn that some bugs and problems are to be expected.
If you're looking for a cross platform Blu-ray player, then Macgo Windows Blu-ray could be your answer.