Unlimited!
Yahoo! Mail just started providing unlimited storage space for free to all of its users making good on their promise. Yahoo! seems to have invested quite a lot to Mail! (contrary to Microsoft which more or less hung Hotmail out to dry) and offers an amazing alternative to Google's Mail. If you haven't used the new Yahoo! Mail yet, now you have another reason to do so.
[Update: Microsoft just announced its new Windows Live Hotmail with 2GB of storage and a new AJAX interface. Sadly for them, it sucks.]