Gmail certainly seems to be the most popular free e-mail service with AB users:
http://mail.google.com/
However, some people don't like the way that GMail automatically scans all their mail in order to append advertising material:
http://www.privacyrights.org/ar/GmailLetter.ht m
I prefer to use Gawab. You can have as many accounts as you link, which you can access both via the web or through a POP client (such as Outlook Express). Details on how to set up an (advert-free) account with Gawab can be found in paragraph 3, et seq, of my post here:
http://www.theanswerbank.co.uk/Technology/Inte rnet/Question407429.html
There are hundreds of other free e-mail providers but most of them only allow you to access your mail via the web. (You normally need to pay a subscription for a service which lets you use Outlook Express or similar POP3 software).
It's a good idea to use separate accounts (irrespective of which provider you choose) for different purposes. Use one for your website, one for internet purchases, one for myspace, one for Friends Reunited, one for emailing your family, etc. Then, if one of your accounts starts getting excessive spam, you can just drop that account (and create a new one for the same purpose) without having to change your address for other purposes.
Mailwasher is also frequently recommended for reducing spam:
http://www.mailwasher.net/
Chris