free sub bass synth ?

[crimsonwarlock]

Taurus might work.

Asynth, also on that page, can drive a serious bassline as well

[bbanyard]

No-one's mentioned DASH-Signature's free little goodie Da Hornet. Makes some very nice noise: http://www.kvraudio.com/get/416.html

[heffus]

starrsynth, pimpin'.

[lobotomie]

anybody mentioned the dx10, which is available for fl studio? has a nice sub bass preset