I have to admit that I've always had a soft spot for Yahoo. I use their e-mail service for my personal e-mail, I use flickr for photo sharing, and del.icio.us for keeping and sharing bookmarks, and Upcoming to post events online. They have a mobile service that lets you do web searches, check e-mail, maps and more.
They have launched a 3rd party platform for developers that allows you to add widgets to their mobile service. You can do something as simple just attach a RSS feed or you can write an XML based application. You do have to use their simplified markup language called Blueprint. It seems pretty straight forward. I'm kind of tempted to write something for it. Unfortunately I don't have a mobile data plan so I wouldn't be able to test it but I seems easy enough you could probably throw something together in a matter of a couple of hours. MySpace is launching a widget that will allow you to post and see your friends moods, browse photos, and send messages.
One of the benefits of this is that Yahoo wrote the platform which works on many different types of cell phones so you don't have to write your application for every different type of phone.