It would be great to have a guitar processor with ardour.
I heard about GNUitar, but i don’t know how good is it.
Check it out: http://www.gnuitar.com/
Marco Ravich
Guitar processor = a bunch of DSP.
LADSPA plugins do all sorts of DSP tasks.
Guitar processor = a bunch of LADSPA plugins
Just stack em’ up as high as you will. 
The CAPS plugin suite, in particular, has a lot of good crunchy amp simulators, etc. Check it out, if you haven’t:
have you tried “creox c” its a kde app.
You can use it through jack so its easy to add it to ardour, or to just play live guitar through it.
somewhat similar to guitar rig, though there are no presets and probably a lot less options (been a while since I tried guitar rig).
I haven’t had the need to use it, but tried it a couple times while jaming with bands and so on.
I’m not a guitarist so I can’t tell if the amplifier simulation is any good or what not, but its a nice option if you need it.
Other than that, yep, DSP!
Hope this helps
Mathieu