The solution, Gmail! Gmail has a relatively unknown feature which allows multiple address to be created on the fly.
If your email address is firstname.lastname@example.org for example then having the dots in the email address doesn't matter and twitter will email@example.com and firstname.lastname@example.org as different email addresses enabling you to set up different twitter accounts this way.
With Gmail you can also append text on the end of your username using the + symbol, for example any email sent to email@example.com will end up in the same gmail account as firstname.lastname@example.org, amazing!
Try it on your own Gmail account. This also works with email addresses that use Google Apps.