The topic pops up quite frequently, so a search in the P&M forum should net you plenty of info. You may have to wade through a few threads to find exactly what you're looking for though.
This thread from the tutorials section should get you started, at least. If you want specific product reviews for 2-part, pourable mold-making silicone (what will allow you to capture greater detail than putties), try searching for some of the more popular names: Smooth-On, OOMOO (might be made by Smooth-On; my memory is hazy on that point), and Alumilite always pop up.