Yahoo Mail is the second largest webmail provider with approximately over 275 million users as per a May 2010 interview with Yahoo! Mail's Carlo Catajan. It offers unlimited mail storage with attachments limited to 25mb per email message. Because of the large amount of subscribers there is also Ymail and Rocketmail which offers the same benefits of a Yahoo mail account. I find the interface to be very user friendly.
You have the option of a preview pane. Unlike Windows Live Hotmail where the preview pane can be on the right side or bottom of the page, in Yahoo Mail it can only be at the bottom of the page. This give you the ability to in effect delete the message without opening it after seeing it. One of the better features is like Gmail, the email message you are composing is saved as you are writing it. However, like Gmail if you leave Yahoo without saving the message as a draft, you likely will lose it. In other words, save your message every so often as insurance of a crash or other reason for you to be knocked off.
Not everything is A-OK about this webmail service. I cannot understand that when you want to delete a spam message you are prompted if you are sure. The other mail services realize if you are deleting a spam message there is no need to prompt to confirm you are sure about deleting them. The drag and drop feature is deceptive. It really does not allow you to move between folders by dragging it into another folder. This however is corrected in the beta version where you can drag a message from one folder to another.
Yahoo Mail works well with Yahoo Contacts. The issue with Yahoo Contacts is that when importing contacts from another application it is not seamless. For a business, the name may end up being the email address. You would need to manually change.
If you currently have a Yahoo email account go to http://features.mail.yahoo.com/ for the beta version. One disadvantage of the beta version is to go to "Options" you need to go to click "Help" and then "Options". The non beta version has a selection for "Options" that is easy to find and you do not need to go to another selection prior. I would anticipate that the bugs in the beta program will be corrected prior to it going live. If you wish to not use the beta version after trying it, you need to go to "Help" then select "Return to Original Mail".
In all it is a solid email program. It has a relatively easy to use interface which is more user friendly than most.