Some of the ways the free e-mail services can make money out of your free e-mail account:
1) Attach a commercial message to some or all of the mail you receive.
2) Attach a commercial message to some or all the mail you send (only possible with Web based services).
3) Display advertisements on the web page you use to do your mail stuff (only possible with Web based services).
4) Sell your e-mail address to commercial organisations.

Your e-mail address is worth money (up to $0.10 per address). However, there are services who promise not to sell your e-mail address. I would recommend only using a service which does not sell your e-mail address. Otherwise you might end up receiving tons of spam (unwanted commercial e-mail).

Bigfoot
Recommended! Your address will be like John@bigfoot.com.

Star Mail
An e-mail address with a domain of your own choice. Examples: barry@star-trek.com, elisa@tina-turner.com

Infospace/iName
Here you can also choose from a variety of domains, like Bill@myself.com, or Karin@cheerful.com.

Hot Mail
Web based. You don't need an e-mail account or e-mail program. Simply access and send your e-mail with your Web browser.

Geocities
Offers free pop accounts and free homepages.

RocketMail
You can use your webbrowser to send and receive e-mail. Many features. Special educational offers.

Wonder Net
Gives you an e-mail address like xxx@wonder-net.com

MailExcite
Web based service. Simple to use.

POBox
Not free anymore. Offers many services though, like autoreply to senders.

NetAddress
Highly recommended! Gives you an e-mail address like benny@usa.net. Also will be having other services, like paging, group addresses, etc.

Lifetime-email.com
Over 200 .COM Addresses to Choose From. Three aliasses provided per account

NetForward
Also offers free web-page forwarding. Does not sell your e-mail address to junk mailing lists.

Cybermail.
User information will never be sold or given out for any spamming purposes.

FreeMail
Mail service from Holland. Try it out if you are Dutch.

Juno
For American users. Toll-free dial-up connection to send and retrieve e-mail.

GMX
Free e-mail service for the German area. No advertisement attached to your mails. Does not sell your e-mail address to third parties.