Believe it or not, some electronic musicians still don't own a 303-style bassline synth. If you're such a person, may we point you in the direction of Togu Audio Line's TAL-BassLine, which can be downloaded for free.
Just like all its rivals, TAL-BassLine is said to be particularly adept at producing acid-style bass sounds and effects. It's designed to look like a piece of vintage hardware and boasts all the controls you'd expect to find on a synth of this type.
Togu Audio is particularly proud of its -18dB low-pass filter, which it says has a warm, analogue sound.