OBVIOUSLY THIS WILL ALWAYS BE INCOMPLETE, FEEL FREE TO PROVIDE LINKS TO ANY OTHER LIBRARIES YOU KNOW ARE OUT THERE THAT AREN'T INCLUDED HERE.

As the title says, this post will give you information about many, many different orchestral sound libraries. Hopefully this will be useful to someone. Some that I have more experience with will have comments, others will not. I encourage anyone who has had experience with any of these libraries to leave a comment, so I can include their thoughts too. The libraries are sorted in price order, from lowest to highest. Enjoy!

Garritan Personal Orchestra
Info: garritan.com, kvraudio.com
Price: $149
While Garritan is not a bad library, my experience with the software hasn't been all that great. It's a good library in that it has a lot of content, but the fact that it lacks a lot of group instruments, dropped in favour of the idea of 'ensemble building' means that if you want to get the sound of 3 flutes playing together, thus you need 3 patches. Doing this for the entire orchestra is not a pleasant process. While strings and brass sorta have 'overlays' which serve the purpose of groups, I don't personally find them that enthralling to listen to, though perhaps that's because I've heard the sounds so many times. The percussion lacks a little too, the snares hardly even sound like snares to me. Finally, my biggest problem, is the lack of velocity layers. The mod wheel essentially acts as a volume slider, there is no difference between pp brass and ff brass, both sound exactly the same, but at different volumes, if you know what I mean. Garritan is a good library for the price, and for beginners with sample libraries, but for most, I'd advise you skip over it. Keep in mind, I haven't tried out GPO4 yet, and I know for a fact that they have included woodwind groups in the new version, but not fixed brass or strings.
Images:
GPO old interface
Ensemble building restricts number of sounds
New interface

Edirol HQ-OR Orchestral
Info: edirol.net, roland.com, kvraudio.com
Price: $385
EastWest Symphonic Orchestra was designed to give you that big Hollywood sound, and it delivers quite well on that. I've never used the library, so I won't comment too much on here. I know quite a few NG users use this library, so if they have any comments, they'll probably end up here.

Morphestra
Info: samplelogic.com, kvraudio.com
Price: $699
To be released on October 1st 2009, Morphestra is bringing something completely different to the sample library scene. You won't find any trumpet, oboe, or viola patches here. Instead, you'll find atmosphere, ambient effects, rhythm timed samples, morphed orchestral instruments and more. The focus of Morphestra is to "blur the line between Music & Sound Design"TM. It is mainly aimed at film and television music. Check out the overview on their site for more details and previews of some of the sounds.

EastWest Symphonic Orchestra
Info: soundsonline.com, kvraudio.com
Price: $195 - $995

Wallander Instruments
Info: wallanderinstruments.com, kvraudio.com
Price: $299 - $599
Wallander Instruments has developed it's own player, the WIVI, which gives you a visual approach to your reverb. Unfortunately, it doesn't seem you can turn off the reverb, making it difficult to mix this with other libraries. KVR says the price is from $299 - $599, but I suspect the prices soar when you buy all their collections. Did I mention they only offer brass, woodwind and saxophones? On the up, the instruments have good velocity layers, but it's not much consolation for a product that seems almost babyish with it's user interface being all colourful with big buttons and such. I've tried the software, I probably wouldn't recommend it, but it's always worth keeping all options in mind.

Project SAM Symphobia
Info: projectsam.com, kvraudio.com
Price: $1399
ProjectSAM's Symphobia is similar to Morphestra in that it's not a traditional library. It comes from a purely cinematic perspective, offering mostly entire sections (strings, brass, woodwind). It also includes many effects, clusters, rips, note bending and the like.
You can try out Symphobia for free at Try-Sound.com.

SONiVOX MI Complete Symphonic Collection
Info: sonivoxmi.com, kvraudio.com
Price: $2995
Moving up quickly in prices now, we have the Complete Symphonic Collection from SONiVOX. I haven't had any experience with this library, though it was recommended to me by a professional composer, so it must be of at least some level of quality.

Vienna Symphonic Library - Special Edition and Special Edition Plus
Info: vsl.co.at, kvraudio.com
Price: $3085
Moving into official 'big gun' territory, the Vienna Symphonic Library is recognised as being one of the most realistic sound libraries out there. The Special Edition includes most important articulations and instruments, and the extension adds more instruments, epic horns, an organ, chamber strings, guitars and so on. The full sample list can be found here.

Vienna Symphonic Library - Super Package
Info: vsl.co.at, kvraudio.com
Price: $30,610
That's right, thirty thousand dollars. I'll let the excerpt from the site tell the story:
This is the ultimate Vienna Symphonic Library anthology, from A to Z. 21 Collections, covering the symphonic orchestra and a lot more. This package includes every Vienna Instruments Collection the Vienna Symphonic Library has ever released, except the Vienna Special Edition. With its unsurpassed number of nearly one million pristine sound recordings, this tour de force is by far the largest compilation of samples on the planet. And still, the power of Vienna Instruments lies in its simplicity, giving quick and easy access to this wealth of instruments and articulations, and rendering authentic results at the simple touch of a key.
Yeah, I just came too.