Security Onion is a distribution of Linux which comes with several forensic, IDS, and NSM tools pre-installed. Although Security Onion is mainly intended for IDS and NSM, it does provide a useful platform for performing forensics, as it comes with many forensics tools installed.
The image can be downloaded at: http://blog.securityonion.net/p/securityonion.html
List of Installed Tools
The following is the list of tools included on the Security Onion distro, as listed at https://code.google.com/p/security-onion/wiki/Tools:
The following YouTube channel provides videos on the installation and usage of Security Onion: