SEM Rush (http://semrush.com http://semrush.com) is a great tool that we use. There's plenty of tools that track your own input keywords, but SEM Rush for it's

SEM Rush (http://semrush.com http://semrush.com) is a great tool that we use. There's plenty of tools that track your own input keywords, but SEM Rush for it's ability to discover keywords you didn't know you were ranking for. Free version is pretty decent to get started.