There are a few different main modelling softwares out there right now.

Amplitude-Its ok, its has the official Fender license to license and supposedly model their amps. The thing that really bugs me about this software is 1, the midi sucks. I use a homemade foot controller and there isn't much to be able to map the controller easily. The other thing with the software is that you can't make a new preset. Everything has to be based off the models and you're limited to the way they chain the effects. The sounds aren't bad, but the software itself isn't very functional or customizable. Its more of a gimic to me. They have the Jimi Hendrix pack, click a preset, sound like this Jimi song kind of thing.

Guitar Rig-Guitar Rig has lots of effects and really weird and neat things like being able to use LFO's and step filters to automate effect parameters. It is very customizable, Midi implementation is good and the clean and low gain models are good. The higher gain models start to sound too digital though and can get muddy. EQ and stuff can save them a bit. The ability to record, loop, playback, play along to mp3's is a nice feature though. I like being able to jam along to something, record it right there, send it to another tapedeck in the software, play that back and overlay on top of that. Its nice if you don't want to fire up a recording software just to mess around.

Peavey Revalver-One that I dismissed at first but am now really starting to appreciate. It does midi good. The models are top notch, especially the high gain metal types like the XXX and the Dual rect models. Other models like that of the Peavey Classic 30 for blues, sound oh so sweet. I like that you can add any VST plugin as an effect anywhere in your chain. I think the tuner in revalver sucks though. For some reason it always says my low E string is in the B range. Never gets right. A small bug though considering the sound. I wish it had the tape deck functions and effect palette of Guitar Rig mixed with Revalver. Though would be my ultimate software. But I've found that I can load up my DAW if I want to record that bad, or make use of the Revalver effects. The other day though I loaded up Guitar Rig as a VST plugin in Revalver just to use an effect quickly. lol.

There are also a bunch of free Guitar Amp plugins as well, some that sound pretty good if you really want to get into tweaking sounds. They are usually only based off 1 amp, and then you need a cabinet sim, cabinet impulses, etc. And for anything like stomp boxes or effects, you need to load them in a DAW. They are not stand alone like the 3 programs above.

I got my copy of guitar rig for free since a guy I knew bought it with the controller, smashed his controller one night in a drunken jam session, and didn't want the software anymore but I'm really starting to lean towards revalver. Most of them have free demos from their sites, so try them out and see what you like the best.
Lefty Strat Copy w/ GFS Lil' killer rails + Crazy wiring
Lefty Kona Thinline Acoustic/Electric
Righty-to-Lefty Silvertone Bass
Righty-to-Lefty Memphis Short Scale Bass

Tascam US-428 USB Interface Mixer
Guitar Rig 3
Nick Crow's 7170 + Kefir + Ryan's impulses =
Guitar Rig/Amplitube =

Don't bother with that piece of crap man! It might seem to sound good at first but just compare it to some of the other amp sims and you will really hear just how much better something like 7170 sounds! Guitar Rig does have some neat effects though