What's the password to the System keychain?

The login (user account) password isn't stored in keychain access, but keychain access automatically adopts the user account password as the keychain access password by default.

So, your password for keychain access is most likely the same as the login password.

There is no default password in mac, the user creates one on install. If your mac was provided already installed, there was likely an assigned password created by the admin, perhaps associated with AD. If you reinstalled OSX, it would not have wiped the password, but if you formatted the harddrive, then reinstalled OSX, you would have created a new password.

You can reset the keychain password as described here:

http://support.apple.com/en-us/HT203192

From the knowledge base article:

... if you don't know your keychain password, you might need to reset your keychain.

Resetting a keychain sets aside the original default keychain file and creates a new one.

To reset your keychain in Mac OS X 10.4, Mac OS X 10.5, and Mac OS X 10.6 Snow Leopard or later:

1. Open Keychain Access, which is in the Utilities folder within the Applications folder.
2. From the Keychain Access menu, choose Preferences.
3. Click General, then click Reset My Default Keychain.
4. Authenticate with your account login password.
5. Quit Keychain Access.
6. Restart your computer.

NOTE: if you reset the keychain, it won't give you access to the keychain you already have.