Yes,
Yamaha Tyro's Roland Virtual Arranger and many other vendors supply expansion sounds for loading. Though I'm not fond of roland as it has very limited expansion sets and one can't buy sounds / patches, etc from third parties.
One word about Kurzweil. It's Andrew Lloyd Weber's instrument of choice for all his theatrical productions. He can't supply a full orchestra for his musicals. In the past he used the k series from kurzweil today he uses the pc3k version
The super orchestral stuff that is add on is mostly for computers not keyboards. Die hard classicalists prefer a very unprocessed sound with is ungodly dry and boring to work with for me. VSL and Miroslav are the big names in dry sounds that you have to spend hours and hours setting up so it sounds right. Hans Zimmer uses his own crafted samples
http://en.wikipedia.org/wiki/Hans_Zimmer
As well as VSL, Atmosphere and Omnisphere
The next is GPO , Garritan is considered the working mans orchestra. It's slightly processed and you have to work the heck out of it to get a good classical sound but it works.
The others worth noting are EWQL and Naitive Instruments, I've always found EWQL bulky the file sizes are huge and hard to work with and yet they have some amazing specialty libraries most notably Hollywood Brass and Hollywood Strings,
Again these are work to setup up right and sound right. You really have to know your way around midi.
Finally the native instruments Kontact libraries Session Strings Pro. This has to be the smartest and best sounding string library I've ever heard. It is highly processed and the cc's make it all the much more realistic.