Another one in the old but gold category. Mammut is a cool sound manipulator tool.
Model F Emu
For some oldskool vibe: Combo Model F is a freeware virtual combo organ, modelled after a well-known combo organ from the 1960s.
LFO
Improved version of the Ableton Live 10 / Max For Live LFO. Get it free from the M4L library.
Creating Multitimbral Sounds
Using the power of bouncing and resampling, you can consolidate your non-playable layers and stacks into one epic, playable sound. Soundbrige explains.
Cynthia Micro
Cool little free VST synth from tesselode.
VHS Audio Degradation Suite
Very nice Reaktor FX ensemble to 80s-fy your songs. Free from the NI Reaktor User Library.
808 Hits & Loops
378 free 808 drum samples from SampleRadar.