Obviously Native Instruments has the sampler world tied up with their Kontakt engine (which is awfully pricey IMHO). And the registration system -- let's just say it's not the best. Some of the official NI instruments I've bought from them work, and some will "say" they're registered but still only be in demo mode which only gives one 15 minutes to work with them.
There are, ostensibly, other options. Plogue makes a free thing called "Sforzando".
As for free libraries there's the Virtual Playing Orchestra and Plogue's own free sounds.