The World Without Us is a book by Alan Weisman that theorizes about what the planet would be like without humans. Check out the graphic on the site and as you hold your mouse cursor over each point in time after we're gone it tells you what would happen.
This concept really makes you think about what we have accomplished and our impact on the world around us, positive and negative. Some really speak to our legacy. For example PCBs and dioxins would still remain after 7.2 million years and the depleted uranium 238 from nuclear weapons would reach it's half life after 4.5 billion years.