Yes, they can as long as they don't break the AdSense TOS with regard to content. I urge you to read the AdSense rules. Also, unless Google have given special permission to the contrary, the ads of only one AdSense publisher can be displayed on a single page. Letting others display your ads is risky because the actions of that third-party could cause your account to fall into bad standing or even be banned. I suggest you only put the code on sites you administer.