If you're one of the many people who don't have a Google mail account, Google is fine with that—but they'd still like you to create an account for their other services. Web apps like Google Docs, Google Reader, and Google Calendar all require a Google account to identify you and keep track of your settings, even if it's linked to someone else's email service. Now, Google is making it even easier to set up an account with them, and keep your e-mail where you want it.
Most people our age got their web mail accounts from Yahoo! or Microsoft's Hotmail service. In fact, Yahoo is the most popular e-mail service in the US, with over three times the number of users that Google has. And while Google has allowed people with e-mail addresses other than GMail account, there's another signup process, and you have to verify your account by logging back into your email and clicking a link sent to you.
But now Yahoo users don't have to worry about all that. That's because Google has started to use OpenID, a standard being used by other web services to provide "digital identity" services, to allow users to create new accounts. Using OpenID, you can now log into your existing Yahoo account from Google to create a Google account linked to your Yahoo email. Google automatically verifies the account, and there's no need to go back to email.
Google is planning to offer this service to Microsoft Hotmail users soon, as well. As more web services adopt OpenID and OAuth, another standard supported by Twitter and Facebook, you'll be able to quickly sign up for new services online using your existing accounts as proof of your identity. The main concern in the long term, however, is that by using a single ID you'll be creating an even more closely-linked web of personal information that service providers will be able to mine.