Amp sims:
http://lepouplugins.blogspot.com/
http://www.theserinaexperiment.net/plugins.html
EQ's, compressors, reverbs etc:
http://varietyofsound.wordpress.com/ (as good, if not better than commercial expensive plugins)
As for free orchestra vsti's I'm not sure there are any, or if there are I'm not sure they'll be very good/usable.