I love free digital instruments and make some of my own and have a big list from all over the net. I've been asked which are the best and this is my attempt to answer that question. It should be noted that I'm just a hobbyist and these are my opinions.
For our purposes instruments will either be standalone VSTs or in the SFZ format. sforzando is the best for playing SFZ files.
The place to start for orchestral instruments is the Sonatina Symphonic Orchestra by Mattias Westlund. He took public domain samples and then worked to make them sound like they were all recorded in the same place. The result is a surprisingly good and coherent orchestra in 500 megs. Its main weakness is the piano, but there are plenty of free ones that we'll get to later.
The original SFZ version is the best and works well with sforzando.
If you prefer I've also done a VST version.
A new alternative is the recently released VSCO Community Orchestra. While it doesn't contain every orchestral instrument and section, it covers the Strings quite well and has some nice solo instruments in Brass and Woodwinds. One great thing is that most of the sustain samples are over 10 seconds. Also it has nice Tremolo for the strings. As I write this it has a beta sfz version that doesn't yet include the percussion mappings, but they are coming. It is a good sized freebie at almost 2 gigs when unzipped and it has almost 2000 samples.
A third alternative is DSK Virtuoso and DSK Overture. DSK also has many other VST instruments. They have nice interfaces and effects, but I've never personally liked the way they sound. Still try some of their instruments and see what you think.
Now that we have a base orchestra it is time to try and find as many solo instruments as we can. Fortunately there are a good solo string instruments.
D. Smolken has done some great sampling and has a sfz Cello and a sfz Double Bass. These are some of the more recent free orchestral instruments and sound very good. They tend to have round robins and several velocity layers.
There is a pretty fair sfz Viola at the bottom of this page.
You'll also see lots of other sfz instruments on the page. Try them out
if you want to have fun. They vary in quality, but some are quite good.
I've done a VST version of the LDK1609 Violin. ldk1609 is a professional violinist who was kind enough to sample his instrument and put them on freesound.org. It has half a dozen articulations and is heavily sampled, though with only one velocity layer.
For an 11 violin section get IKM's Sampletank 3 taster. It also has many other instruments including a great piano that will be discussed later.
Unfortunately free brass and woodwinds solo instruments are more difficult to find.
I've used the University of Iowa samples to make VST instruments of 3 flutes, a trumpet, a tenor trombone and a tuba at the page I've mentioned before. Scoll to the bottom.
A gentleman named Anthony Deaton has created has created Anthony's Phil Harmonik with a flute, an oboe, a clarinet and a bassoon using the same samples.
The Sampletank 3 taster also includes a nice flute. It has a sax, trombone & trumpet trio as well, but sax isn't exactly classical.
I guess if you like flutes you are in good shape!