Raw video (MP4 etc.) as Parcel Media or Media on a Prim under Windows

Raw video needs QuickTime. Not every 64 bit viewer supports it, though — the Singularity Viewer is rather the exception in that it does.

A note on QuickTime under Windows:

Apple has discontinued it, and there are two security holes you have to be aware of if you use it, because they allow malicious code to be injected.

See: information on QuickTime at heise.de

Flash-based video (YouTube etc.) under Windows

Most viewers need the matching Flash Player plugin in the Firefox version here.

Only the CEF-based viewers will presumably use the Chrome version.