Why won't OS X trust GitHub's SSL certificate?

this worked for me:

Keychain.app > Preferences > General > Reset My Default Keychain

UPDATE

A less drastic option is to delete the DigiCert certificate from the login Keychain: you should already have one in the root keychain, anyway. This error appears to occur when the two do not match.

There is a new problem as of July 26h, 2014 when an old, apparently quasi-wide spread certificate expired.

Based on https://www.yesthatallen.com/fixing-an-old-digicert-issue/

Instructions for clearing expired DigiCert SSL certificate on OSX

1. Launching Keychain Access via Spotlight
   • ⌘-Space
   • Type "Keychain Access"
   • Hit return
2. Ensure expired certificates are shown; enable "Show Expired Certificates" in the "View" menu.
3. Search for "Digicert".
4. Right-click the certificate with a red X and select "Delete DigiCert High Assurance EV Root CA"
5. The certificate may not look removed until Keychain Access is restarted
6. Restart your browsers