A highlighter pen is something that I will always have in my makeup collection. My favourite is the YSL Touche Eclat or Radiant Touch. I love the luxe gold packaging (even though it attracts dirty finger marks) and the pen dispenser makes it nearly foolproof. I usually wear this underneath my eyes, over the top of some concealer, or just on it's own if I'm feeling lazy. I apply it in a triangle underneath my eye (see here for a demo by Zoe Foster) and dab (never wipe) it in with my ring finger. I love this product as it gives a really natural look to my makeup and it's super easy to touch up (not to mention, it makes a big difference). I find that this product does crease under my eye but if I set it with a translucent powder it stays perfect forever.
This product is on the expensive side ($55) but it's worth it, one tube of this stuff lasts for aaaages. I have bought some for about $30 from Chemist Warehouse before, but after comparing the packaging with ones bought from YSL I'm not 100% sure about it's authenticity so I will be buying from YSL in the future. I've heard that Maybelline do a product which is a great dupe for this so I will probably try that one out once I've finished the one I own now. If anyone has tried this out I'd love to know!