This is a great free tube amp modeller. It only models one amp, and doesn't have a huge amount of control, so it won't be useful for every project, but the sound quality is great for what it does (it certainly is up there as one of the best freeware amp modellers). Its based on a Marshall amp and gives some good heavy sounds. I wasn't able to get much I liked in the way of clean or crunch sounds though.
The gain and drive work together to control the amount of distortion, while the four other controls serve as a basic EQ.
I needed to put a noise gate before-hand to stop it amplifying the noise from my guitar, and you will probably want to add your own reverb / delays etc afterwards as it has no built in effects. Cabinet modelling is not necessary, as this has already been taken account of (apparently a 4x12 cabinet emulation is included).
It doesn't come with any presets, but there isn't too much to tweak so its easy to explore. It serves as a great advert for their commercial more fully featured amp modeller.