I sign up for them all, just recently, in order to create new aliases everywhere, and I'm not sure why I'm doing so.
Primarily, I am using Yahoo! for quirks and friends, and Gmail for signing up to site subscriptions and acquaintances.
I found out about
GMX - Free E-Mail - Webmail & POP3, IMAP, and although I have signed up for it, I still have yet to integrate it fully. I think it is some sort of e-mail address book, and it could help verily much if I wasn't so lazy to test it out.