In pro audio there are any number of devices and even more software products that emulate tube distortion. Some actually use tubes, but most use DSP to achieve their sound. They are widely used as an effect.
A company call Line 6 has a very well received line of guitar amps that DSP model a variety of tube based guitar amps. They might not exactly nail a specific amp, but they get pretty close and can sound quite good in their own right.