From the MTV show Jackass, a skull with crutches. This one must be nearly as popular as the classic symbol by now. Also, notice the missing tooth.

To start with, one of my favorites: A Storm Trooper with crossbones.

Ok, so this one is the classic symbol we all know. I consider the above image the most prototypical version of the skull-and-crossbones symbol, thus all variations are more or less derived from this.