I bought a few of these banggood LEDs in order to help avoid me constantly losing orientation of my hexacopter. They were very cheap and very easy to fit. The video doesn't do them justice but flying at about 2pm on a clear day they were still well visible from the ground at a reasonable distance. Definitely recommended.
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These look like high quality, but only available in Australia
http://www.volo.systems/leds.html
https://www.youtube.com/watch?v=KTMwoJcdQL0
Never tried these, but I love the ones from Hansen Hobbies: http://www.hansenhobbies.com/products/lighting/fls/
Bruce did a nice review of these a week ago. Promising
https://www.youtube.com/watch?v=HtPK9MgEHdo