Proteus

For sheer amount of amp/pedal models this thing has, it's a must have.

I'd go as far to say, that with this installed, you probably won't need any other guitar amp and pedal emulation.

I tried a dozen or so models and all of them sounded good and quite close to the physical stuff it's been modeled after.

Also big kudos to the author for taking extra effort putting up a Linux versions and tackling all of the most popular plugin formats.