Backup contacts on phone

If the phone supports the SyncML technology and you trust Google, you can configure address book synchronization via www.google.ru/mobile/sync/.
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This solution for obvious reasons, is platform agnostic and can be adapted to any OS.

Android... Well you poneli.

On Windows Mobile I use pimbackup and plus sync with Google, as said above.

On the Symbian used the OVI service, very convenient.

Opensync is

In my opinion, there is a server option, which must be supported by almost all modern phones using SyncML.
I was not engaged, and can't say anything about how comfortable it is, etc.
But, this technology is a plus — it allows you to sync contacts with the server remotely, via GPRS, etc.