Truecaller, Free Caller ID for your Nokia 5800! (Sort Off)

Truecaller is another totally free awesome app. It runs in the background and automatically runs a reverse phone lookup for those numbers not saved in your contacts to give you information on whose calling.

Note, for this app to work you will need to have caller ID provided by your service provider and it will use data (about 1kb) if you're on the go.

However, this app can also be used as a reverse phone lookup over wifi at home.

Download here or from the Publisher's website.