The traditional reference mark is the asterisk *. The lesser known reference marks are much nicer — the dagger and double dagger. Hoefler and Frere-Jones have a blog post about reference marks, with examples — the ones on the right are from one of their new font, Sentinel.
They’ve written about other punctuation marks — see my blog post about those.
And in case you’re wondering about the title of this post, it refers to the anatomy of the dagger: the quillon is the guard that separates the hilt of a knife from its blade, and the choil is the notch where the blade meets the quillon.