One widely used system (running-specific, not HR-based) is Jack Daniels' Vdot calculator, which is on AP.
http://www.attackpoint.org/trainingpaces.jsp
Enter your time for some race distance you did recently (road 10K or half marathon would be good), then click on Training to find out recommended paces for various types of workouts, including different types of intervals. Click on the workout types to get an explanation of what Daniels means by each one as well as its purpose. These paces aren't designed for trail running, skiing or our other sports but it gives you an idea of the range of training paces that can provide benefit.