Wet is (1) an adjective meaning covered or soaked in liquid, and (2) a verb meaning to make wet. Whet is a verb meaning to sharpen or to stimulate. The latter has origins in Old English, where it related to sharpness and sharpening, but in modern usage it’s confined almost exclusively to the phrase whet [one's] appetite. Because it’s more or less forgotten outside this phrase, whet is easy to confuse with the far more common wet.
Here are some images of the freshly painted wall to whet your appetite for the 2011 season. [SFist]
The campaign to whet the public’s appetite for the clothes began in September during New York Fashion Week. [Sydney Morning Herald]
Like any good overture, the work served to whet the audience’s palette and raise excitement for what was to come. [Daily Breeze]
Wet your whistle
The reverse error—using whet in place of wet—is rare, but it is surprisingly common in the idiom wet [one's] whistle, which means to take a drink. Whistle here is a metonym for mouth, so the phrase literally means wet [one's] mouth, and it’s easy to see how this came to mean what it does.
This sort of error is common:
Whet your whistle with “the finest tea this side of China,” on the east side of the main market hall. [Patch]
This is correct:
Just when you’d thought you’d be able to wet your whistle on Sunday, a Republican caucus killed a measure to allow the sale of alcohol on Sundays last week. [Walton Tribune]