What systems/mechanics have proven most effective at giving each player about equal spotlight-time and enforcing that equality between look-at-me players and 'turtles'? There's a pretty shy player who will be participating in a campaign I'm going to run, and we're also going to have some loud personalities playing alongside them. I am looking for systems that mechanically lend themselves well to having all PCs be solidly enmeshed in the problems and plot of the campaign (possibly some kind of explicit star-of-the-scene rotation system or in-world complication counter) or a specific mechanic that encourages this that could be grafted onto other systems.
Related to this question, but I'm curious if there are crutches out there that could supplement good GM social practices. Obviously there's no substitute for the latter, but I'm interested in what aids are available.