Guitar Tuner or Guitar effects Algorithms?
Best book on the topic is probably http://www.amazon.com/DAFX-Digital-Udo-ouml-lzer/dp/0470665998.

There is some freeware stuff (very mixed quality) here: http://www.musicdsp.org/

Many companies spend a lot of effort in making good sounding effects and well working tuners to make money with it, so access to "good" non-commercial sources if fairly limited.