According to money.CNN.com, two of the best online sites for retirement planning are AARP.org and the United States Government site MyMoney.gov. Since AARP (formerly the Association for Advancement of Retired People) is an organization devoted to retirement-aged people, their website is a top-rated resource.
The United States Government site is helpful for determining how to apply for Social Security benefits and answering questions regarding protection of benefits. Money-CNN's website also provides a significant amount of information including a Getting Started section that seems quite thorough. They also recommend that those considering retirement review the websites of their brokers for additional planning tools.