For the cost of a decent plug in you can find an Otari 5050 tape machine (1/4" 2 track) on ebay. Don't buy one that is selling for like $50 but try and spend a little more for a unit in good working condition. This way you won't have to spend too much on maintaining the the machine. You can buy "one pass" tape for pretty cheap (tapetape.com) and use it about 20 times before you'd need to buy some more...
The great thing about having a tape machine is that you can bounce stereo sub mixes to it, re-record it and line up your tracks in PT. Or you could mix down to it (or both). The Otari is a great sounding machine for the $ and worth having. Learning to align a machine is very simple, although you may want to buy one that has an MRL (a test tone tape) so you won't have to buy one seperate, I believe a 1/4" MRL is about $90.
So there, that's my opinion on the matter. There is no plug-in that I've used that can really give you realistic tape saturation, this may change in the future as programmers get more in depth and computers become more powerful, but for now analog tape is still where it's at.
Later,
musik