Windows Mobile

Windows Mobile in its original format is pretty useless, but if you can get your hands on a modified version then it begins to excel as a business phone.  It only works well with an Exchange account of which they are many suppliers where you rent an account and normally get a licenced copy of MS Outlook thrown in.  This means that all your emails, contacts, calendar and todo items instantaneously synchronise between your phone and desktop and vice versa with no extra software.

The problem for most people is that they have a windows mobile phone with the default settings which is based upon the same principles as a desktop.  This does not apply to a phone and the interface needs to be simpler and usable with a finger instead of using a stylus.  IphoneOS obviously has moved the goal posts and the up and coming Android OS from Google will as well.  But some of the mods to Windows Mobile are worth a look.