Funny: Amazon (and Newegg) were actually the sites I thought of that do this very well. I can shop around, add things to cart, get recommendations based on my shopping history, everything really, until checkout time. At that point it becomes reasonable to make an account.

If Amazon did what a lot of sites do that I come across -- that is shove a monsterous sign-in form first thing then start asking me for referrals -- that would be a total turn off.