First of all, let us get one thing absolutely clear-there is no such thing as a spam-free email service. Whether you choose Yahoo, which probably has the best spam filter available for a free service today, or a premium, paid service, it is only a matter of time before the spammers get to you. What you can do, if you are careful, is to limit the damage that occurs. To do this, you first have to realize how spam reaches you.
Spammers get hold of your email ID when you post it at various websites where you register, such as mouthshut, or when you are required to give it to download some software. Now the best way to overcome this, and a way that has worked admirably for me for the last 8 years, is to use a second web-mail account purely for entering in such websites. I have a sify ID, where I get over a hundred junk mails a week. I just go in once a month, delete all the mails, and let the spammers start all over again! As a result, I get only one or two junk mails into my main ID every week.
Another way your ID gets spread about, and one over which you dont have much control, is by viruses that attack your friends’ emails and mass mail themselves to everyone in the addressbook in addition to sending the addressbook back to its creator. The only way to beat it is by educating your friends about using good antivirus software and software that protects you from adware (eg: Spybot search and destroy).
If you are using Outlook or similar software to access the web, using Spamhilator or similar software may help. Remember, spam-free email is not a service, it is what you make of your email ID.