Software Synthesizers

Can't afford or make space for a hardware synth? Not a problem with modern technology. Please try not to shit up this list with gimmicky synths, make sure that they're actually useful before adding them.
Many of these are available for at least Windows/Mac OS X. Linux compatibility is less common but highly appreciated. These are listed by price for convenience (but you're probably just gonna torrent them anyway aren't you?)
== Poorfag Tier (Free) ==
* Dexed (Win/Mac/Linux)A Yamaha DX7 emulation: 6 operator FM synthesis. Can load and edit DX7 patches to send to a real DX7. However, FM synthesis may not be the easiest for the beginner. As the somewhat popular Native Instruments FM8 falls into the same category (and can also play DX7 patches) you could also learn a lot from FM8 tutorials, of which there seem to be a lot more than DX7 tutorials.
* Synth1 (Win/Mac)THE classic freeware synthesizer. Synthesis is modeled after the Clavia Nord Lead 2, however the interface not so much: 2 oscillators, FM and ring modulation + modulation envelope, 2 LFOs and added effects the original doesn't have. The default bank is a GM set and pretty boring, however there literally thousands of free presets out there.
* TAL Noisemaker (Win/Mac)Modernized version of the TAL-Elek7ro subtractive synthesizer with 2 oscillators and 1 sub-oscillator. Clean and nice user interface makes it very good for beginners.
* U-he Tyrell N6 (Win/Mac)Modern subtractive synthesizer with 2 oscillators, 1 sub-oscillator, 2 LFOs, ring modulator and modulation matrix. Sounds as good as a paid U-he synth.
* U-he Zebralette (Win/Mac)Stripped down version of the U-he Zebra (see: Richfag Tier) synthesizer with only one of its oscillators. Still pretty useful and sounds as good as the big version.
* SQ8L (Win/Mac/Linux)An Ensoniq SQ80 emulation, which was a hybrid analog/digital synth from the 80s. 3 oscillators (AM+sync for the first two), lots of waveforms, 4 LFOs and something like a modulation matrix. The presets sound and feel like something out of the 80's, and it also implements the original 80's user interface. Comparison the the sound of the original.
* OBXD (Win/Mac)Emulation of the Oberheim OB-X, OB-Xa and OB-8 analog synthesizers from the beginning of the 80's. Very old school subtractive 2 oscillator, 1 LFO, 1 filter. Has a really good sounding filter section.
== Entryfag Tier ($50-150~) ==
* Karma FX Modular (Win/Mac)
* NI Razor (Win/Mac)
* U-he ACE (Win/Mac)
* Sugar-Bytes (Win/Mac) - Makers of extremely unique sound modules with unconventional interfaces
* D16 Group (Win/Mac) - They make sound modules that are mainly based on vintage Roland gear like the 303, 808, 909, SH101, and 606.
* Audiorealism (Win/Mac) - Another company that makes sound modules based on vintage Roland gear like the 909, Alpha Juno, and 303. 
== Richfag Tier ($150-250~) ==
* DCAM Synth Squad (Win/Mac)
* U-he Diva(Win/Mac)
* U-he Zebra(Win/Mac)
* Xfer Serum(Win/Mac) 
* IK Multimedia (Win/Mac)
* Tone2 (Win/Mac)
== What the Fuck Tier ($300+) ==
* Omnisphere (Win/Mac)
* Sampletank (Win/Mac) (Price varied with option)
* WAVES Plugins (Win/Mac) 
* Native Instruments (Win/Mac)