Native Instruments Guitar Rig is a modular effects processor for guitar and bass (and for vocalists, in my opinion). The Player version is free. Its modular and expandable features are very nice, you can choose among about 30 free presets – you can buy more for adding effects, but the default ones are more than enough – each of them editable by:
- adding one or more components (filters, delays, compressors, …)
- deleting one or more components
- changing their sequence
I tried it on mac using guitar and my voice, it plays really good. On this site you can find clear explanations and the downloading page. Pay attention, the whole procedure is a little bit intricate, follow steps exactly.
I suggest this software for geeks.