Following are 5 online tools which will help you backup your mobile phones data which includes Contact book, Calendar, Photos, SMS’s, etc..

In order to use most of the services listed on the page you will need a mobile phone based on SyncML standards with WiFi, Edge or GPRS enabled phone.

First in the line is ZYB.com
ZYB service is compatible with most of the phones as it is based on SyncML standard and works for more than 400 handsets. It also lets you backup your data by sending SMS’s to  ZYB. ZYB also offers an online phone to phone migration tool for transferring contacts and everything else from one phone to another and they call this service "The ZYB Migrator", access it here.

Anywr.com
Most of the services on this article work on SyncML standards, so does Anywr.com. This service goes a step further and lets you sync with online mail services such as Gmail, Hotmail or Yahoo. It also offers  Messaging and micro-blogging and much more.

Bloove.com
is a little different from other services. It does let you do everything other services offer but the user has to install a mobile software from http://bloove.com/m(visit from your mobile phone) to be able to use the services of Bloove.com

Mobical.net
Mobical is not any different from the services mentioned above. It works on the SyncML standards as all of the other services and does backup the phone all the data on the phone.

Mobyko.com
Mobyko works only through SMS [Update: Correction based on Lain's comment] with SMS and SyncML like other services. So in case you want to backup your contacts or photos or anything else on the phone you will have to either SMS, MMS or your phone should support SyncML.