The Center for Election Science promotes cardinal voting methods, i.e. those where candidates are rated on a scale, as the fairest and simplest. We focus our reform efforts primarily on approval voting--the simplest cardinal voting method and, arguably, the simplest voting system of all.
Approval voting is a single-winner voting method that allows voters to vote for any number of candidates. The candidate with the most votes wins.
Score voting (sometimes called range voting) is a single-winner voting method where voters rate each candidate on a scale. The candidate with the highest rating wins.
Take a look at where score voting and approval voting are used today.