Just another Warning sigh with exclamation mark. I made it symmetric with some gradient. The file in wiki (http://commons.wikimedia.org/wiki/File:Warning.svg) is not so symmetric.
The symbol for radioactive hazard, following the official proportion specifications (radius R for center circle, 1.5R to 5R for blades of trefoil). Done in Inkscape.
One of the famous yellow warning signs: explosion danger. Note that there are several signs with explosions - there's also "explosive atmosphere" and "explosion risk/explosive materials/compressed gas"...