Additionally, the ARIA Engine has many sound sets that are also free. Sforzando itself includes some free samples (scroll down that page to the Sforzando section, and is the 'Sound banks for sforzando' link on the far right). Not all keys are mapped for some instruments, so it is a royal pain to find which keys truly have sounds associated with them. I like Sforzando for a couple simple reasons, it will tap into installed programs that have the sfz file structure, and shows you which keys are mapped for a given sound bank. The engine to it is the ARIA player, and there are more free libraries people have made for those.
Sforzando is a free sfz player (the format used by Dimension Pro and other instruments).