Free version of the Alpha VST synth. Basic interface, good sound quality though. Download from KVR.
The Music and Sound Design Platform (also referred to as Music_SDP and MSDP) is free and open-source software built for electronic and electroacoustic music performance, production, improvisation, and experimentation. Music_SDP is a DAPE, or Digital Audio Performance Environment, functioning similarly to a DAW (Digital Audio Workstation), but designed with a special modular structure built for live performances. MSDP provides a non-linear, infinitely expandable, totally hackable, easy-to-use interface for sound design that encourages and rewards exploration and experimentation. It is a great platform for beginners and professionals alike, and provides all the tooling and control needed to make wild new sounds, build complex sonic projects, and execute flawless performances.
MFreeFXBundle contains a set of 30+ audio effects, which are completely free (You can extend the features of these free plugins at any time by purchasing a licence).