This may or may not be related to the player. On Windows, it depends on the set of codecs installed in the system. There are players not relying on that. Besides, as you mention ASP.NET in your tags, it may mean that you need not just the player, but some streaming solution.
Please see open-source VideoLAN:
This product is used as player, converter and the streaming software, at the same time, in combination, or separately. Practically, it does nearly everything. And it does work with FLV.