Little Snitch alerts you to outgoing network connections.

A firewall protects your computer against unwanted guests from the Internet. But who protects your private data from being sent out? Little Snitch does!

As soon as you're connected to the Internet, applications can potentially send whatever information they want to wherever they want.

Sometimes they do this for good reason, on your explicit request. But often they don't. Little Snitch allows you to intercept these unwanted connection attempts, and lets you decide how to proceed.

Little Snitch informs you whenever a program attempts to establish an outgoing Internet connection. You can then choose to allow or deny this connection, or define a rule how to handle similar, future connection attempts. This reliably prevents private data from being sent out without your knowledge. Little Snitch runs inconspicuously in the background and it can also detect network related activity of viruses, trojans and other malware.

WHAT'S NEW
Version 2.3.4:
New preference option to enable reverse DNS lookups in Network Monitor.
Improved performance.
Improved Installer application (more robust in non-standard environments).
Improved compatibility with older PowerPC machines running Mac OS X 10.4.11.
Fixes an issue with the "Confirm with Return and Escape” preference option.
Fixes an issue that could cause a kernel panic on machines with 64-bit kernel.

Requirements
PPC / Intel, Mac OS X 10.4 or later.

Note: Tested only 12 hours but not tested the full 72 hours

Link for more information: http://www.obdev.at/products/littlesnitch/index.html