Google has taken it’s Web based Gmail service (still nominally in beta) and shrunk the interface so that it can be conveniently used on mobile phones with Web browsers. Details are here and the actual service is at http://m.gmail.com. If you don’t want to punch that in on your phone’s browser, they have an application where you enter your phone number and they’ll send you a text message with the URL.
The beauty of this is that since Gmail is a web application, there are no charges beyond whatever your phone plan charges for Internet access and there is no problem, common with other mobile email systems, of your mail account getting out of synch, since you are just accessing the regular Gmail using a different, smaller interface. Other features include viewing attachments and “reply by phone” to email whose senders are in your Gmail address book.
Check out the FAQ for which phones have been tested and more details. The service is currently English only and I have seen press reports that say it is limited to US mobile phones only, although why they would bother with that limitation isn’t clear and I coudn’t find it in the documentation.
You do have to wonder why no other provider of Web email has done this as well. Yes, it costs money to develop and test the interface for many different small screens and many phones don’t yet provide web browsers or Internet connectivity, but the latter is only a matter of time. Windows Mobile phones have a somewhat similar setup for MSN Hotmail, but the whole point of a Web based interface is that it should be OS independent.