redsn0w 0.9.15b3 for Mac was available to download from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The actual developer of this free Mac application is iPhone Dev Team. The program belongs to System Tools. We recommend checking the downloaded files with any free antivirus.
From the developer:
A cross-platform jailbreaking, unlocking, and customizing tool for iPhones and iPod touches. Customizations include boot logos, recovery logos, and "verbose" boot. It's a standalone program that doesn't use iTunes (no custom IPSWs are involved).
Apple Firmware 4.1-5.1.1 supported. Hardware supported. iPhone 3GS iPhone 4 iPhone 4S iPod Touch iPad iPad 2 iPad (3rd generation)
You may want to check out more software for Mac, such as Significator for iTunes, which might be similar to redsn0w.