Usability 
I been looking at the various mobile web based email providers. Previous posts covered Gmail Mobile and Microsoft's new Live Mail Mobile. Today it's Yahoo's turn.
Yahoo has had mobile email since 2000. The current version has been around for at least a couple of years but it's a good design and is aging well. There are both WAP2 and WAP1 versions of Yahoo Mail. The interfaces of the two versions are almost identical. The screenshots show the WAP2 version. The only difference I've found between the two, is that you can add the sender of an email to your address book with a single click on WAP2 but not on WAP1.
The Yahoo mobile mail interface is simple and intuitive. The first time you connect to mobile mail you will have to login. Yahoo saves a cookie on your device so future logins will be automatic . You can also log out if you concerned about security should you phone fall into the wrong hands.
After you log in, the initial screen lists your mail folders showing the number of unread messages in each. Numeric access keys (which are used throughout Yahoo mail) make selecting a folder a single click operation. In the list of messages only the message title is shown. This a trade off, by not showing the sender's name more of the title can be shown. It would be nice if you could choose between displaying sender, title or both in the list but that option is not available.
Once you open a message, the sender's name, title, time and date is displayed above the message body. WAP2 users can click the sender's name to add the sender to their address book. This is handy but there is no obvious way to get back to the message from the address book other than the browser's "Back" key. The "Options" link lets you reply, forward, delete, move the item to a folder or view headers. The "View Headers" option is unusual in a mobile mail app and shows the reply-to field, other recipients and the full sender address. 
At the bottom of every screen in the Mail application is a menu (last image) which allows you to compose a new message, create folders and check other POP mail accounts (which must be set up in advance on the desktop). Composing a message is straight forward - pick recipients from the address book, key a title and body and hit "Send"
While there's nothing flashy about the Yahoo Mail mobile UI, it is efficient. The underlying Yahoo Mail feature set is one the best of all the web-based mobile mail services. Unique to Yahoo is Free Intellisync software to keep your Yahoo address book in sync with Outlook, Outlook Express, Lotus Organizer, ACT! and Palm and Microsoft PDA's. The software can also sync most of these applications with your Yahoo Calendar and NotePad. You can make phone calls or emails directly from the address book on your phone which makes the synchronized Yahoo address book particularly useful if your phone can't synchronize with the desktop or if you have hard time keeping your contacts up to date across multiple phones.
You can also set up Yahoo Mail to pull mail into your Yahoo inbox from external POP servers. SSL connections to POP servers aren't supported so this won't work with Gmail.
I really like Yahoo's mobile alerts. An alert is a SMS sent to your phone when new email arrives. You can set filters to only send SMS's for emails meeting certain criteria - such as a specific word in one or more of the fields; sender, to, subject or body. Hotmail and Live Mail Mobile also have mobile alerts that work similarly. Gmail doesn't but you can get the equivalent functionality by setting up your mail to be forwarded (unconditionally or based on a filter) to your phones email-SMS gateway address AND also selecting the option to keep a copy of the mail in your inbox. I find that mobile email alerts give me an almost Blackberry like email experience on even the most basic phones. The SMS is my "You have new mail!" indicator - telling me to go into mail on my phone's browser and read the new message. It's really pretty cool. I do have free incoming SMS, if you have pay for SMS and get much mail you will want to use filters so you only get alerted for the really important emails.
Yahoo, Live Mail and Gmail are all good mobile email services. Yahoo stands out for it's desktop address book synchronization and the access to email on POP servers, Gmail for it's awesome speed, huge email storage limit and the ability to view many attachments on the mobile and Live Mail has a fresh and powerful new UI. To me the most important feature of a mobile web site is it's speed and Gmail is the speed champ by a wide margin which is why it's my top pick.
Update 16-Sep-2007: This describes an old version of Yahoo Mail which no longer exists.