DroidWall v1.4.5 (free)
Requirements: ROOT REQUIRED!!!
Overview: Front-end application for the iptables Linux firewall. Allows you to restrict which apps can access the network

This is the perfect solution if you don't have an unlimited data plan, or just wants to see your battery lasting longer.

Recent changes:
* Auto-refresh app list
* See blocked packets destination on log
* Tablet UI support
* Fixed "Exit code: -1" error
* Many other bug-fixes

Download:
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