If you're talking about DX/VST plugins for your sequencer/DAW then here's what I use:
IK Multimedia Ampeg SVX - models of various Ampeg rigs and stomp boxes
IK Multimedia Amplitube - guitar amp sims and stomp boxes
Native Instruments Guitar Rig - as above
BBE Sonic Maximiser - BBE Sonic Maximiser in software form, works just like the hardware unit
Ohm Force Predator - NASTY distortion
Waves MaxBass - Produces harmonics over your original bass content. Makes your ears think low down bass is there when it isn't. Useful for remastering demos for the web, e.g. Myspace, where people use PC speakers to listen to your tunes.
Crysonic plugins - they make a bass plugin similar to above and various unique compressors, eqs and enhancers
Compressors and EQs - Waves API & SSL collections, Sonalksis
PSP Vintage Warmer - The best vintage tube compressor plugin I've heard. It can really warm and dirty up your sound, it's incredible.
There's also a good audio to midi covertor I've used that's responsive enough for live use (just can't remember the name of. it!). Wicked for triggering a 303 or any other synth you can think of from your bass. Eats an AKAI Deep Impact for breakfast!
The great thing about these plugins is that they can replicate some mega expensive hardware and if you know where to look you can get them all for free! On a try-before-you-buy basis, obviously.
