ReQall is a very nice piece of to-do software; it allows you to make a to-do with reminders via a wide range of media. When setting yourself a to-do note, you can call their toll free number and dictate the note (using voice recognition software), text the note, instant message (IM) the note, send via a Firefox extension and by applications available for many phones (including of course iPhones and Blackerrys).
There is integration with Google Calendar & Outlook, sharing of items with other people, and the premium version even has location-aware reminders (ex: you go to the supermarket and want to be reminded to buy milk).
The basic version is free and the Pro version is $2.99/month or $24.99 a year – a real bargain for what you get.