For a guitarist, nothing produces tone like an amplifier with its power output tubes driven into clipping.  There's something about even-order harmonics that is pleasing to the senses, that you can't quite get from transistors or digital signal processors.

This page represents interesting links I have found regarding tube amps.  Most are links to manufacturers which I think make cool products and/or have useful information on their sites.  I'm primarily interested in guitar amplifiers, and I hope to actually build my own tube amp one day...