Plugins

There are several websites that offer (free) guitar amp modeling programs and (guitar) effects such as EQ, compressor, delay, etc. for download. 

Here is a list of a selection.

Tip: If you would like to learn more about amp simulation, including how it compares to an analog amp, take a look at Dominik Schempp's bachelor thesis "Software-Ampsimulationen – Eine Untersuchung der Signalkette und ihrer Defiziteanhand einer Musikaufnahme" (Software Amp Simulations - An Investigation of the Signal Chain and Its Deficits Based on a Music Recording) (in German).