I’ve spent a while trying to write a concise response, but fail miserably every time. Suffice it to say that Kontakt is the dominant force in the market. And as such, it will return the most results.
There is no alternative to Kontakt for playing kontakt libraries. (except for really old libraries from K1, K2 and K3).
But on Pianobook, many of the contributors have included not only the library in their preferred formats (nki/nkm, exs, sfz and others) but also the .wav files that you can use in other samplers to create or edit yourself.