This chart shows, if humankind disappeared right now, how long it would take for various artifacts of our presence to also disappear. By far, the most permanent of our marks is that of nuclear waste and debris. For example, the depleted uranium weapons we are using in warfare are completely irresponsible and idiotic. Here is an article on NewScientist.com that spells it out a little more.
One of the first things to be rendered useless would be the very words that I am typing now... along with the rest of the Internet!