It's worth investing in this plugin because I guess being a guitar player it's probably the best one I have invested in. There are some free ones out there that are fairly good though. I have a few listed on my site (click here) go to the guitar tab.
17 amps, 27 cabinets, 54 effects
Legendary guitar and bass amplifiers, modeled with award-winning Dynamic Tube Response Technology. These amps convey the character, charm, and feel of their real-world counterparts. Each amp has its own matched cabinet plus an extra set of alternative cabinets – 27 exquisite models in all.
Although the free ones are not what I normally use I will say that some of the free one have some really awesome presets. So take advantage of the free ones. There are plenty free ones but the ones I have listed are what I like the best.
Here are some other great plugins as well.
Amplitube (click here)
Peavey Revalver MKIII (click here)
Line 6 Pod Farm (click here)