I'd say strictly speaking, no. Especially if the site or page carries a copyright notice. That said unless it's of great commercial value or literary merit, most times you'd get away with it.
The easiest thing is to ask permission to use it. I've done it many times for the wiki. If you state it's a not for profit, non commercial site, and offer a credit (although in the case of the wiki we don't offer links), most people are only too pleased to help.