email, e-mail, E-mail

Here’s a usage note from Dictionary.com:

The transition from World Wide Web site to Web site to website as a single uncapitalized word mirrors the development of other technological expressions which have tended to take unhyphenated forms as they become more familiar. Thus email is gaining ground over the forms E-mail and e-mail, especially in texts that are more technologically oriented. Similarly, there is an increasing preference for closed forms like homepage, online, and printout.

Dictionary.reference.com/browse/website.