A couple of months ago, I found VirtuAmp, a (real-time) virtual guitar amplifier that comes with a range of virtual pedals, including a tremolo, flanger and compressor. It works actually pretty good too and it’s easy to setup: your (desktop) microphone can work as an input, but preferably you should use your (semi/electric) guitar cable and plug it right into the line-in/mic of your computer (for this you may need an adapter of course).
Since VirtuAmp processes everything in real-time, you may need a fast processor. You definitely need a good soundcard (my onboard cheapy ass freaking soundcard makes a mess out of it). Additionally, if you feel so obliged, virtuAmp comes with complete sources.
(I’m also not sure how this compares to the real stuff, but hey, it’s fun anyways).