http://iwl.me. Simply put in a few paragraphs of your writing - from a blog, journal entry, letter, e-mail or chapter from your great unfinished novel - and see how you compare to the pros.
I put in a recent column and got King. For grins, I entered a second column and got David Foster Wallace. Hmm. Not exactly reassuring. King is a weird duck and Wallace, tragically, hung himself after a lifelong battle with depression. Still, I'd trade royalty checks with either of them in a minute.
Try it yourself, and be sure to tell me under Comments who you write like!