I just love Wikipedia, I really do. Where else would you find an interesting, well-referenced, useful, and pretty comprehensive list of common misconceptions? I knew about most of these already, but I did learn a few things: I apparently had an over-simplistic idea about how the Christian canon was decided, there are 128 mutations at human conception (1.3 of which are harmful, on average), horseshoe crabs have blue blood because their blood is copper-based, and shaving does not cause hair to grow back thicker.
There is, I admit, one serious problem with Wikipedia, nicely illustrated by xkcd:
(Hat tip: Dean)