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Audjoo Helix – Hybrid synthesizer
This synth is HUGE. The GUI may be not the easiest, but this is one of best sounding free synth i’ve catched in a long time. Just listen to the mp3 player on the site. Unbelievable. It features four oscillators with 16 voices unison, a wave table shaping technology for spectral morphing, dozen of filters and modulations. This synth is declared as beta, meaning that the product may change during time even if stable. Available for both Windows and OSX. UPDATE: Not free anymore

EMU Proteus VX - Streaming Sampling Synthesizer
An high grade sampling synthesizer featuring the enhanced Proteus X Composer Bank. EMU Proteus VX can be used as standalone or as VSTi. This software is not copy protected or card locked, as other EMU software instruments. You need to subscribe EMU newsletter in order to download the software. Even if declared some users reported that VX will install and run with Windows Vista, this app instantly crashed mine so… save all your jobs before run this software on a Windows Vista based PC.

Native Instruments Kore Player – “Super instrument”
Based on the same technology of Kore 2, this software brings you 300MB of production ready sounds. It includes six Native Instruments engine such as KONTAKT, REAKTOR, MASSIVE, GUITAR RIG, FM8 and ABSYNTH. The player can be expanded purchasing Kore Soundpacks, which are pre-made sounds that can be solely loaded into Kore Player or Kore 2. Unfortuntately, so far, there is not a way to create custom soundpacks. Kore Player is available for both Windows and OSX.

Tascam Continuous Velocity Piano – Spectral morphing acoustic piano.
How about morphing the spectrum of a sound instead of massive multisampling? A very clever idea applied to this great free software from Tascam, in order to use a single velocity sample expanded from pp to ff. Just a taste of what we have to expect in the near future from Tascam and, i hope, other manufacturers.

Togu Audio Line Bassline – SH-101 style substractive synthesizer.
What to say: great GUI, great sound, great price. Available for both Windows and OSX, this nice plugin will stay for years into your collection ready to pump up your productions. Remember to take a look to EVERY (juicy) product availeble on this site.

UVI Workstation – Software workstation instrument
This seems to be IS a GREAT tool for your personal collection. A full featured software multitimbral workstation, just like a Korg Triton or a Yamaha Motif. You can load sounds, split them, layer them, add dozen of FX and so on. On the same page you can download the 415MB Soundpack Demo with a lot of ready to play sounds.
Check out this video:

Yellow Tools Indipendece Free – Software Sampler
Premioum sound library of 2GB totally free, based on the Indipendence 2 Audio Engine. You have all the features of other Indipendece softwares, but audio import function is disabled. Commercial use of this library is allowed and it is available for both Windows and OSX.

Special mention to de la Mancha for its great sounding as well almost unusual free plugins. Don’t miss basic64 or truc.