"Bass Machine"

I figured this was the best area to post this sort of topic, but anyways:

Lately, I've been looking at ways to work on jamming and improvisation. I have a drum machine program (I use Hydrogen) and a recording program (Audacity), the only problem is that I'd really like to be able to create bass lines (and even other instruments if possible), so I was wondering if there were any free programs out there that are "virtual" bass guitars or synths. I found one but it was expensive and unfortunately I don't have the money to spend on it. I assume that since Hydrogen and Audacity are also freeware, a freeware synth or bass simulator might exist.

I tried to record basslines from PowerTAB files, but it didn't really sound that great. Likewise, I tried to play a bassline with my guitar and lower the pitch with software, it also didn't sound very good. So any help would be greatly appreciated, thanks!