Last weekend was the Larimer Square Piazza del'Arte. That sounds pretty uppity though - I prefer to call it the Sidewalk Chalk Art Festival, although that a misnomer since it's neither on the sidewalk nor confined to just chalk.
It's amazing what these artists can do on such a rough canvas as Larimer Street. At least put downon a base layer of water-soluble paint to give a better background. Other just put down huge amounts of chalk.
It's really a great show and free to boot. If you live in the area, you should definitely check it out. I've gone for the last three years in a row and haven't been disappointed yet. In fact, I'll even close with a few photos from last year's show.