In addition to the solid informational links
below, the hidden-gem is the section featuring
actual images of recently purchased diamond rings on James Allen's
website. Now you can look for the exact style of ring you're
thinking about buying and get a realistic picture of how it will
look, without all the flash. This is very cool and helpful feature
and I highly recommend checking out some of the recently sold
rings--see who's buying what!
If you're looking to delve further into the world of diamond knowledge, you can also check out GIA's website (Gemological Institute of America). GIA is a non-profit company whose goal is to help educate consumers on all-things-diamond. They are the #1 place in the world for unbiased diamond grading and evaluation--the fact that the engagement ring I purchased came with a GIA certificate went a long way in getting it appraised so high.
Also, if you're interested in some more diamond and engagement ring reading material, you can check out some of these various articles on the subject. They can be very helpful in your quest for the perfect ring:
And of course there are the obligatory James Allen and Blue Nile reviews from various mystery shoppers.