Gah! I didn't notice that Mario had already commented on the
Solid State Symphony.

Tracy Collins - of Indiginus - is also the person behind
Amadeus. That's built from older pre-existing samples that were licensed from various sources. It's a fun library if you're looking for something that's complete and works well together, but I think less realistic than other orchestral libraries. It's also got symphony patches much like
Solid State Symphony, so that part is sort of a "realistic" version of
Solid State Symphony.
Here are some of the best free (or almost) free orchestral instrument libraries I've found. Not enough to build a complete orchestra from, but certainly useful for supplementing what you've already got.
Orchestral Sampling has a
lovely free trumpet that's got an orchestral feel, but could probably be used in a pop production.
There's also a
nice free orchestral trumpet from Norrland Samples.
Orchestral Tools has a very nice
legato french horn. They have their own player, but if you look for the fine print, you can find the Kontact player. I
think it's about $2. (I can't see it because I already bought it, and I'd have to sign up to see the price).
Strezov Sampling has a free
Afflatus Minimalist Violins Free that has a lovely, delicate sound.
Performance Samples has a free
Solo Legato Violin and a
Solo Legato Cello, both of which are fantastic... in the right context.
VST Buzz is hosting the
Pocket Blakus Cello, which is a great cello with some serious crunch on the bottom end. It required a signup to download, but it's worth it.
Here's a link from a
VI-Control forum member that doesn't require a signup, but apparently doesn't have the VST wallpaper.
Fluffy Audio has a free
Simple Flute that's not that detailed, but works, especially if you have a breath controller. It's probably the weakest instrument in this list - it's hard to find a good free flute.
Hephaestus Sounds has a
free oboe that's quite good. You use the mod wheel to change articulations - something I haven't seen in other instruments!
Cinesamples has a free
Mister Rogers Celeste that's probably better used for intimate recordins, but is
technically an orchestral instrument.