Guitarix

Guitarix is amazing. I just want to point out that that Mac and windows comparability comes with a giant asterisk. They are both branched from ancient forks of the source code and there are no binaries so you have to compile them for win and Mac, a task beyond the pale for most potential users. I did a double take when I saw Windows compatibility on kvr, but the label is misleading here.
As for the plugin on Linux, it is the best most versatile amp modeler out there and Linux, unlike windows, can actually do real time, so I have run guitarix on Linux on a computer from 2009 at 32 samples at 48kHz sample rate. To give some context, that means 0.67ms latency, which is like instantaneous with the speaker moved 8 inches further away from you. It is a whole different feel from using a 128 sample buffer.