Embracing users with multiple personality disorder
As people use the web and the available services more and more, some users sometimes need more than one account for a single service. However, switching between two accounts is annoying: first, the service's "remember me" functionnality is useless. Second you have to log out and sign back in, re-entering your password everytime (unless you don't mind saving your passwords in plain text on your computer via the browser's "remember passwords" feature). Third, there's an easier way and all it costs is some developer time and a rethinking of the traditional account.
One account, multiple profiles
What I suggest is that upon registering on a website, one account is created. The only information associated with the account would be what is needed to contact that person and to login, so a name and an email adress (or the OpenID account used to open the account for example) would sufice.
Then the user needs to create at least one profile (or persona/personality/...) to start using the service. Of course, the first profile creation could (and probably should) be created at the same time of the account so that the user can start using the service as soon as possible.
The user should then be able to create as many profile as wanted and switch between them easily (see mockup further down). Ideally, the account would be invisible to other users, only the information in the profile would be visible.
Of course, not all services would benefit from having such a feature, but I feel like most would. I know of one company that manages the accounts in a way that is similar to what I'm suggesting and it's 37signals. At the end of 2009, they noticed (and probably experienced first-hand too) that users needed multiple accounts for their services and had to log out then sign back in to switch between them. So they did what I'm suggesting now and introduced a single sign-in across all their products.
Other websites that would be great with this feature include Gmail/Google, reddit, Twitter and your favorite social networking site. Imagine being able to switch between your real profile and a number of other novelty/fun/secret profiles on which you wish to remain anonymous.