Keyboard shortcut to prevent login items from loading but still allow extensions
Disable automatic login in system preferences - users and groups - login options. 

Reboot

At the login window type your password and then immediately hold down shift. 

This will disable login items on a per login basis. 

With proper timing this can be accomplished even with automatic login enabled. You must hold shift as soon as the spinning gear stops. 

If you are talking about Lion Resume, that's another story.