Mixpanel and KISSMetrics are 2 of the most popular customizable analytics tools out there. Both tools allow you to put sniplets of codes into your pages to send events to Mixpanel or KISSMetrics to track. They're also strong tools to track how valuable your users are, how they buy, and which pages give you the most value.
For example, you can customize a funnel to view the efficiency of your conversion funnel - See how many visitors come, visit the signup page, sign up, and use your main feature. A example of a chart from Mixpanel looks like this.
It can show you your efficieny every step of the way for a customer's entry to the website, to signing up, to using your key features, and to paying for your service. With the data from here, you can know where you need to improve to get more users to do what you want them to do.
These tools are quite easy to use, and both providers have very awesome instructions to get you started and optimizing your website.