I've used Rescue Time, but unfortunately, they don't offer the (free) ability to track time across your browsing, only showing that you were using a browser during that time.
I spend a lot of time browsing, for both productive and unproductive reasons. It'd be immensely useful if I could find a plugin/program that could track the sites I go to and how much time I spend across each.
And perhaps this is reaching, but the ability to see some sort of timeline, graphs, and/or plots of browsing usage over a time period would be brilliant. But the bare essentials is a tool that tracks the time to specific sites.