DroidWall is a powerful front-end for your Android device’s Linux firewall iptables. Clocking over a million installs on the Android market, it’s probably the most popular — and was at one point the only — Android firewall app which allowed you to granularly control which apps could access the network.
DroidWall requires root access on your device, and has advanced features that allow a user to manually define custom iptables rules. DroidWall also allows logging and setting custom scripts on startup and shutdown. It's useful to lock down your apps and take control of what apps can connect to the Internet.
Google Play download | Developer site