Native Instruments Komplete 5

* Kontakt 3 - This has probably the industry-standard sampler for a few years now, and comes with a 33 GB library. There is a free upgrade now on NI's site to the 64bit version, which if you've got a 64bit computer, will help a great deal. Tons of 3rd party software is written specifically for Kontakt (Tonehammer, Cinesamples, etc). While I prefer the PLAY engine used by EastWest, Kontakt has been around, and is very stable.
* Guitar Rig 3 - Basic amp modeling and distortion box. Newer amp modelers are out, but this one sounds great.
* Massive - Big soft synth, aimed mostly at huge base and beefty sounds. Has to be played (heard) to be believed.
* Reaktor 5 - This alone used to go for more than $400. It's an extremely diverse, and complex modular synth that's impressive sounding, if daunting, and a programmer's dream. Lots of 3rd party progs for it.
* Absynth 4 - Very lush and deep synth once developed by Brian Clevenger. Great atmospheric sounds and pads. But it can do more than that. Much more.
* Battery 3 - Dedicated drum sampler with a ton of kits, plus it's fairly easy to build your own.
* Akoustik Piano - Really three grand's, and an upright. Better pianos are out there, but this is a very good staple for any kit.
* Elektrik Piano - Four electric grands in one. Mostly Rhodes based.
* B4 II - Dedicated to recreating the sound of the Hammond B3 organ. Of all the soft synths, this one sounds the most real, if not quite as "playable" as a real B3. Still, if you're looking for "that sound" it's here.
* FM8 - Probably still the best FM soft synth out there. Just one great FM sound after another after another from this beauty.
* Pro-53 - Based on the Sequential Circuits Prophet-5, out of all the synths here, this is the most disappointing to me. It's not as good as Arturia's soft synth (Prophet-V), but does come with a ton of Prophet like patches that sound good in a mix.