Komplete is suite of software which includes all kinds of great Native Instruments virtual instruments. What I love about it is that it really includes just about everything I would ever need to use when combined with the effects that come with REAPER (my DAW of choice.) And it's all really quality stuff too.
If you're like I was and you're looking for an "all in one" solution to all of your sampler, synth, drum machine, and guitar amp sim needs then I definitely recommend the latest version of this software suite (Komplete 6 as of this writing.)
Komplete 6 Features
Kontakt: Includes samples of just about any kind of instrument you can imagine.
Guitar rig: Amp sims and effects for guitars or bass guitars.
Battery: Drum machine with a huge selection of drum samples. Highly tweakable.
Synths: All kinds of different synths including Absynth and FM8.
Akoustik: Four different acoustic pianos which sound great.
With this post I want to emphasize important it is to use a weighted key MIDI controller such as this one (and in my research it's the best deal around for a full 88 key weighted keyboard - that's why I bought it.)
Really there's not much point in even getting Pianoteq unless you have a weighted keyboard because what makes it special (and more realistic sounding than sampled pianos) is that it is extremely responsive to the touch of the keyboard player and that touch is only going to be right if you're playing on weighted keys.
That's not to say that I only like using the SL-990 because of Pianoteq. I love the way it feels on everything. I think it helps give the sound of my virtual drum machine (I have been using Superior Drummer 2) a more "real" sound and I even like the feel of it when playing VSTi synths (some seem to disagree on this, but I like it.)
I think people often underestimate how important velocity is in giving music it's "feel." Real people don't play every note or every drum hit or every piano note with the exact same velocity so when we have static velocity in our MIDI it gives our virtual instruments a very "robotic" sound.
So much of the "groove" is created by the differences in velocity between the different notes. It's those differences in velocity that makes the music come alive. It's what makes it human. And I think the SL-990 Weighted Key Keyboard is a great way to get your "humanity" into the MIDI in your DAW.
If you've never played a weighted key keyboard, it's definitely something to check out.
Pianoteq is a different kind of piano sampler because it's not actually a piano sampler at all. Instead it's a piano "modeler." I don't know exactly what that means technically but what I do know is that it sounds more realistic than samplers do to my ears and it also takes up a lot less space on the computer!
On top of the very realistic sound that you can get with this VST instrument, there's also the great variety of ways that you can adjust the sound (you can change the tuning, the mic position, you can choose from a large variety of different pianos.)
But what you really want to hear is an example of what it sounds like. That's why I'm including Beethoven's "Moonlight Sonata" as played by Pianoteq below. I've actually only got Pianoteq 2.3 (Pianoteq 3 is the latest version and it's what's available now) and I still think it sounds amazing. In fact, I think it sounds so good that I haven't bothered upgrading to Pianoteq 3 even though it's supposed to be even better.
***EDIT***
Due to a bogus copyright claim by WMG, the PianoTeq version of "Moonlight Sonata" was taken down by YouTube. Apparently it sounded so realistic that they thought it was actually a recording of a pianist playing "Moonlight Sonata" that they have under copyright. I think that's actually pretty good proof of how great PianoTeq sounds.
Obviously the song itself (being a Beethoven composition that is about 200 years old) is not under copyright, so the only explanation is that they thought the recording itself was under their copyright. Of course this wasn't actually a recording at all. It was the PianoTeq software "playing" the song.
***END EDIT***
***NEW EDIT***
The (stupid) ban on the video seems to have been lifted, so now you can hear it again:
***END NEW EDIT***
I recommend the SL-990 weighted MIDI keyboard to take full advantage of the power of this piano modeler software. A huge part of what makes Pianoteq sound so real is that there's basically an unlimited amount of variety in the velocity (which is different from samplers which have a limited variety as far as velocity goes) and of course you need a weighted keyboard to take advantage of that aspect. The SL-990 is what I use and I love it.