And I don't even know where to start looking. I was thinking it might be an egg cluster, but can't find anything useful without knowing at least what kind of organism made it.
After a lot of scrolling through image searches I stumbled upon the answer: this is the egg mass of some sort of horse-fly (Tabanidae). Almost identical egg mass here: http://bugguide.net/node/view/21171
I'm assuming I won't get a more specific answer than this.