What are the most valuable techs for trading purposes?

I don't typically trade techs with the AI very much, and I think this is a weak point in my game. From browsing around the forums, it appears Education, Philosophy, and perhaps Aesthetics have good trade/bribe value with the AI. Are there others I should know about?

I'd also like to know more about what determines an AI's willingness to trade a tech to me. What determines which techs they're willing to part with? And what determines how much they want in return for a tech?

Oh crap. Wrong forum. I dunno how I wound up in CivRev. I don't even own it. Could a mod delete or move this thing to the regular strategy section for Civ IV?

I usually try to maximize my science after 3000 bc.
My favortite tech to sell is the alphabet, after I discovered writing.
Literacy, and the dead-end irrigation is also great for selling. But do not sell Literacy to soon, you want to get the next tech in that diection, so that the AI never gets the chance to catch up in the techrace.
This often makes the AI put more focus on developing tech, instead of getting military units.
And since you are still deleoping tech faster than the AI because of your hard efforts to do so, the AI will reap few benefits from their efforts.