Here's one that closely imitates Raven's Progressive Matrices and claims to have been calibrated with a sample of 250,000 people: http://www.iqtest.dk/

Here's another one: http://sifter.org/iqtest/ . I can't find any mention of where the questions came from or how it's calibrated, but it's shorter and doesn't require Flash.

Neither one asks for an e-mail address or any identifying information. They might be too easy for some on LW, but harder ones tend to cost money. As Viliam_Bur pointed out, any free online test's validity is questionable, but the first one is basically a direct copy of a "real" test, and neither one has any apparent ulterior motive. Anecdotally, they were both within 10 points of each other and my "real" score.