I seem to remember a link to a website being posted here once, that was to a site that would take a block of text and evaluate how well-written it was. Basically, you would enter a few paragraphs into the calculator and it would return a value of what the level of writing was (i.e. grade school level, high school level, college level, etc.). I’ve been trying various searches of these boards, as well as Google, but so far no luck finding it. Did I just dream this website/calculator, or does such a thing exist?
Microsoft Word does that; if you do a full spelling and grammar check, you get various readibility statistics. Does that help?
Something like a Gunning Fog Index?
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Are you referring to the Flesch-Kincaid algorithm? That calculates reading level by American grade levels. Though for my money Phillip Chalmers’ scoring of the Gunning Fog algorithm is much more accurate and much more fun. 
Juicy Studio have a form to test the readability of websites. It gives you the various scores under different tests though I assume that it also includes things such as links. I guess you could always just post a block of text online and link to the site (I’m assuming you wish to test for a longish piece of writing and not just forum posts. If the latter then I’d also use MS Word).
There’s an online text evaluation site here where you can enter blocks of text and have them analysed.