From the archive. Originally published on Neel's World in Sep 2013. Kept here as part of the blog's history since 2009.
True Caller is a very good free app available for Android phones which helps in identifying unknown callers through the use of a huge database of mobile numbers generated from user submitted data. The app shows up the caller details in the home screen fetched from the internet when connected to 3G network during an incoming call. One can also set blacklist of unknown/known callers to get rid of spams. Search the Play Store for download options.