[heisenzombie]

The old-school method to do this on purpose was a smear of Vaseline on a glass filter in front of the lens!

[Sharlin]

Another less-messy way is to stretch a (piece of) pantyhose in front of the lens.

[Waterluvian]

Now I finally understand that one Simpsons joke.

[lancesells]

In the colder months I would exhale onto the lens giving me a varied look to this kind of blur. The great thing is it wouldn't last.