These days many people access multiple accounts at once. Sites like Facebook, for example allow you to log into most news sources with your information from them. Some new services called aggregators also take advantage of this ability, allowing you to use one password for all of your messengers, emails, and newsfeeds. However, if you are going to use these services it is best to use safety measures.

Convenience
Logging into one account then making comments onto other sites is common practice. Many news sites give you the option to sign on with another service, such as Twitter. Doing this removes the need for you to juggle multiple Internet accounts.

This process is called single sign on authentication or SSO. When you have one log-in active you do not need to repeat the process. If you choose to sign out of your accounts, you can do it with one single step logging out of your main account. You probably already use this in your daily life if you spend any amount of time on message boards or social networking sites.
Recently, with the introduction of multiple instant messaging programs, came a selection of email/chat aggregators. Digsby, Trillian, and Pidgin are three of the most popular ones. These programs can be downloaded to your computer and then you make one log-in. After you add your messenger and email accounts to the program it uses the single sign on authentication you made for it to check these services.

Safety
If your main username and password fall into malicious hands that person has the ability to access all of the accounts that you have tied to it. This can have a wide range of impact on you, depending on the type of information that you have tied to your account.

There are steps you can take to limit the risk of someone finding out and using your single sign on authentication. Make sure that you regularly change your password instead of using the same one for an extended period of time. When you change your password try not to use something that can be easily tied to you, such as a birthday, significant other, or pet's name. And finally, check and make sure that you are using the safety measures that are provided with the service that you are using. Many sites allow you to tie in a phone number or a separate email address with your log-ins. While nothing can protect you 100% from theft, these steps can greatly lower your potential risk.