If you have a Google account, you can use their Calendar service at http://www.google.com/calendar
. When setting up events in your calendar (e.g. you can set up a monthly reminder for any direct debits, credit card bill dates etc), you can opt to get notified by SMS before the event is due. You can choose how many days/hours/minutes before you would like to get notified.
I have found this to be a life-saver when it reminds me of various bills that are due out of my account, so I can be sure there is enough balance. You can also use it to get SMS reminders for birthdays, meetings, anything which you can add to your calendar really.
Best of all the SMS alerts are free!