Yoyo stoppers are usually fitted 2/5 of the way down the leading edge of the kite to catch the lines when you do roll up (yoyo) tricks - they stop the line sliding down the LE and snagging the wing tip.
Lams 'skyclaws' have been out for a couple of years now and are supposed to work well. Although I did at first think all those sharp pointy bits might damage the sail when it is rolled up in transit in a kite bag I've yet to hear any horror stories so they musy be OK. They do look rather cool!
Some kites - Benson, JoE and a few others - use roll bars instead to get the same result.