Bigroomkixsynth is a free VSTi specifically created for making Kick Drums and Bass Drums. It uses no samples only 100% synthesis to achieve a thick meaty UMPH to your track with ultimate control to the sound.
The sound is split into 4 sections, SUB, BODY, TRANSIENT and HAT/NOISE with lots of controls to fine-tune each characteristic of the layer. It has a compressor an EQ boost and a Key Select. You can even change the background colour. The Synth is free to use and share.