Welcome to CAcert.org
Install Cert OS X
# Download the certificates
curl -k -o "cacert-root.crt" "https://www.cacert.org/certs/root.crt"
curl -k -o "cacert-class3.crt" "https://www.cacert.org/certs/class3.crt"
#
# CAUTION: Verify the certificate fingerprints before proceeding!
#
# Import the certificates into the desired keychain
sudo certtool i "cacert-root.crt" k=/System/Library/Keychains/X509Anchors
sudo certtool i "cacert-class3.crt" k=/System/Library/Keychains/X509Anchors
# Clean up after ourselves
rm "cacert-root.crt"
rm "cacert-class3.crt"