Recently, navigating the Debian repositories I found Rakarrack, an Open Source Linux program to simulate various guitar effects in real time. This sound effects are often done with expensive physical sound processors, but this application makes it all with only one cheap computer (well, I suppose that the quality will rely a lot on the sound card).
Each effect is a module and and has its owns parameters which can be adjusted with sliders from the GUI. Multiple effects can be chained in different configurations.
For the audio input and output, It uses the Jack Sound Server. Initially Jacks is a bit complicated to set up but is very versatile. I use the qjackctl to configure and run Jacks simplifying this task. First we start qjackctl and from its GUI, start the jack server. Then run Rakarrack, and with qjackctl we connect Rakarrak to the system sound output and input:
The latency (the sound delay) of Rakarrak is quite good, but depends a lof of the Jack server configuration. This snapshot is the configuration that I am using from qjackctl: the Latency is the important value, and can be minimized adjusting the “frames by period” parameter:
I played a lot using different presets (it has lots of pre-configured) and there are some quite good, but others does not sound as good as a physical pedal, maybe also by le low quality of my notebook sound card.
It also has an integrated guitar tuner, but for this purpose I prefer Lingot from my Friend Iban Cereijo.
A lot of years ago I tested Guitar Rig, a quite pretty (and expensive!) windows applicattion for the same purpose, but then the latency of the sound was too high. Rakkarrak is simpler but enough for most non-professional guitar players, and I like it a lot!