Like what was stated above, Boobass if you want a bass guitar for free. It isn't that great but it's better than you can expect for a freebie.
SHREDDAGE 2 might be a pretty damn decent option if you have some spare money laying around for not only a good bass sound but also a Lead & Rythem sound. It's Orientated more for Lead & Rythem but with a bass amp/cab VST it packs alot of punch. It's a Kontakt 5 library but the best part is that unlike the Original Shreddage which seemed to only run in a demo mode in the player, the new one is fully playable and supported in Kontakt 5 Player which can be downloaded for free off of the Native Instruments website.