I use Ozone 3 (
www.izotope.com) which includes a harmonic exiter with differents forms of soft saturation and distortion...just like tape sat. I don't know alot about howto use it, but i usually saturate the middle frequencies of the track (300-10000) a bit, and maybe a little in the top too. Is some fatness is needed i delay it with a couple of milliseconds. I usually do this in the stereo imager though.