I purchased Komplete 10 Ultimate earlier this year and have been learning my way around the content. Would love to hear about what user content from the NI site people have found useful to them and why. I write in a wide variety of styles so nothing is really out of bounds. Thank you all in advance.

I don't think it's in the User Library but Boscomac has some absolutely superb Donationware (i.e. donate if you like) ensembles for Reaktor. There's synths, modeled instruments, drum machines and effects. They really are a top notch labor of love kind of deal and I use them all the time to the point where I did actually donate to him. Can't recommend these highly enough. 
 
http://boscomac.free.fr/
 
To be honest I haven't checked out the User Libraries for anything except Reaktor. There's a ton of great stuff for Reaktor though, I have loads of it installed but I haven't found a use for all of it. What I love about the Reaktor User Library is that you get some really original, quirky sounding effects and instruments. One of my favorite soft synths ever is Eccentrix, FM wave mangling heaven. Great once you get used to how it works. There's loads of good drum machines and sequencers too. I quite like The Silverbird Live.
 
Other good synths and instruments I've found are Polaris, Acoustring and Serenade (an amazing modeled violin). There are many others which I don't at this time recall 
 
 

Thank you for the suggestions sharke. Much appreciated!
 

I'll second the Boscomac suggestion. Amazing stuff, really.

I'll second the Boscomac suggestion. Amazing stuff, really.

Most of the good stuff isn't in the NI User Library, it's scattered all over the web. Searching for it is half the fun.
 
Here are couple lists of sample freebies: 
http://www.theflipsideforum.com/index.php?topic=11431.0
http://earmonk.com/free-sample-libraries/
 
Here's a KVR sticky that's been running for about 8 years and though disorganized it touches on just about every free sample library: http://www.kvraudio.com/forum/viewtopic.php?f=42&t=188856
 
A good source for small, very inexpensive sample libraries is Sampleism.
 
Some commercial vendors that offer high quality stuff for < $50: Indiginus, Wavesfactory, Bolder Sounds, Lyrical Distortion, Hollow Sun, Embertone, and lots more I can't think of off the top o' my head.
 
Nearly all sample vendors offer freebies to generate traffic to their site and demo versions to entice you to purchase. Impact Soundworks' Trial Pack is one example. A new one I've just picked up from Cinematique is a free demo version of their very cool Ensembila cinematic sequencer.
 
Bargains abound, even at some of the higher-priced vendors' sites. SampleLogic sells a collection of their products for $25 called the TryPack that's recommended. Indiginus has some libraries for $5, that are prototypes for what later became full products but are useable as-is. SoundIron has their Iron Pack series that sell for 5 bucks or less, or all 12 of them for $49.
 
If you're into pianos, Sampletekk is the place to start for low-priced pianos (sales are frequent).  For cheap drums, there's Drumwerks and Analogue Drums (grab the freebie Big Mono). For acoustic guitars, see Indiginus first.
 
Of course, this is all about cheap and free stuff. Once you're ready to spend some dough, the sky's the limit. Orange Tree Samples, SampleLogic, Spitfire Audio, Kirk Hunter, VSL, get ready to mortgage your house and sell your kids.
 

Great layout Bit!

I have downloaded a couple of hundred Reaktor ensembles from the NI Reaktor user library, and about half of those I ended up keeping.  Some of those are excellent, and some not so much.
 
I relied on the rating values other users gave the synths I was thinking of downloading, and I think that helped me skip some dogs.
 
I checked out the other libraries and there are not really any sizable numbers of things like Kontakt libraries or Massive or the other stuff to download.  As was noted above, there are lots of freebies and low-cost libraries for Kontakt out on the web, and I have also found TONS of other freebies, like for FM8 I found every single Yamaha DX* anything factory patches out there in a single collection, and FM8 can directly read the DX* sysex format and play those sounds, so I have tens of thousands of DX* patches that sound FABULOUS in FM8.
 
If you want those, send me a private message and I would be happy to send them as a series of 8 emails with an attached zip folder per email to give you the collection.
 
I also have several REALLY nice Kontakt libraries that were completely free, for various instruments, and it IS fun to hunt on the web for those.
 
There is an expansion pack available through the Cakewalk store for something like $20 (I think) that would also give you every single EMU Proteus sound released all in one collection (might be $49).  I used to have 3 Proteus rack modules back before computers, and loved those sounds.   They offer it for Dim Pro, BUT that same collection is available in Kontakt format too.
 
One of the BEST Kontakt libraries I got off the web was a MASSIVE percussion library, which was free, once I made a $10 donation for something or another.  It is a HUGE library with tons of high-quality samples.  They even released a free update for it about 6-8 months later.
 
SOOOO - shoot me a PM with a real email address, and I  can send you some goodies (along with those FM8 DX* banks).  I will even throw in a couple of surprise Reaktor ensembles I had built for a couple of specialty sounds I wanted.  They are REMARKABLE.
 
Bob Bone
 

After watching some demos of the Wavelore(?) pedal steel using Kontakt, I started thinking I would have to set money aside and get the full Kontakt. So I'll wait for a sale and maybe get Komplete (not the Ultimate) in the deal, though it sounds like you guys actually prefer the free libraries, which is amazing.
 
Anywhere in particular that Kontakt deals happen?
Thanks
Michael
 

You can wait for the yearly Summer of Sound or go for the regular upgrade from a Kontakt approved library, Michael.