I noticed this question was asked a while ago, but wanted to contribute for anyone that comes by.
Most of the time you will come across 6%, but there's no set or standard commission rate across the industry.
Depending on the location and potential selling price of your home, you absolutely can negotiate commission rates. Even with top agents. Keep in mind that working with a reputable agent that can sell your home for the most amount of money is better than finding any agent that will lower their commission rate.
The key to negotiating is to compare multiple agents, and interview all of them. We streamlined this process at UpNest (http://www.upnest.com) by creating a marketplace where top local agents compete against each other to earn your listing. When agents compete, they bring out their most competitive commission rates and value added services. You also get the satisfaction of "comparison shopping" for agents.