Ignite Amps Emissary
One of the best metal amp heads I've heard...
...and it's a free VST plug-in. This one in particular is clearly aimed at metal players, but with "metal" being such a wide term these days, its versatility comes almost by design. I use it mostly for Black Album-style rhythm tones, and it gets a lot of use in leads too. Along with LePou's HyBrit plug-in, it's one of those products that'll make you go "how is this free?!"