I don't know why this particular misspelling seems to be so common, but here it is:
sneak peak.
Peak is the top of a mountain. Peak is the apex.
Peek is the proper word. Peek means to take a stealthy look at something.
I've been seeing this pop up frequently on websites (including those of magazines, which ought to know better) and I don't know where it came from. But it's becoming one of those tiny things that drive me nuts. On the annoyance scale, it's the spelling equivalent of hearing someone masticate.