Do pets relave stress? And if they do, what kind of a animal does it best?

Ha, they also can be a cause for stress too. I'm stressed out with my cats right now because they just.. annoy me sometimes - meowing at the door to get out at all odds of the night, and fighting with each other (I have two girl cats.. who usually get along) usually at night while I'm trying to sleep. *sigh*. They've been stressin' me out lately. But I do agree that they can, at times. When my cats are cute and behaving well and just want to be pet or want to cuddle - then it kind of alleviates my stress.