The free TubeDriver, a tube amplifier emulation plug-in created by Nick Crow is my favourite for some time now. It even beats expensive well known plug-ins.
Why is this thing so cool?
When you turn the Bias parameter to the left you get some sweet odd and even harmonics. The high end sounds just right, not sharp so not much upper harmonics are generated. When you turn it to the right less even harmonics are generated. Turned fully to the right the signal is without even harmonics. It truly sounds like real tubes.
The EQ is super useful and the oversampling algorithm is great for keeping this device to sound as analog as possible. It even has a 16 x oversampling option on rendering which is simply brilliant! Mono and Phase options are also available.
Windows (VST 32 and 64bit) and Mac OS X (VST and AU 32 and 64bit) versions are available at the developer’s website. It’s free but donations are welcomed.