There are a lot of review sites out there. http://www.dpreview.com is a good one… however, my favorite is http://www.slrgear.com.
In truth, they really only do lenses, but it’s an incredible site. The most in depth reviews of lenses you can imagine, including an interactive 3D graph showing you the focus profiles at any given aperture/focal length. It’s hugely entertaining to play around with the 3D graph and see where the sweet spot is for the lens and where it starts to really break down… as far as sharpness and vignetting goes.
For example, I’ve got the Canon 50mm 1.4 lens. You can see looking at the SLRgear graph that at 1.4, the lens really isn’t that great. But once you get to 2.0 and especially 2.8, it’s a great lens. It’s a good thing to check if you want to get the most out of your lenses.
Here’s a screenshot: