A good concealer is one of the most important products you'll ever own (that and a foundation that gives you the skin of an 18 year old). I've tried many in my time, and have to say that I'd rate Alison Raffaele's True Concealer amongst the top. It's £18 from Cult Beauty, and has an amazing creamy consistency that glides on to the skin and then blends away to nothing.
I think the magic is in the ingredients, and this concealer is jam packed full of wonderful things, from Vitamin A, C and E; to pomegranate and reservatrol to fight anti oxidants. Somehow it also fits aloe in (for soothing) ad sodium hyaluronate (for cell regeneration). But leaving the ingredients aside, the reason I now keep it in my makeup bag is that it manged to cover up some hideous circles when I went ton all night bender- and people said ho well I looked! That's proof in my book.
It's available in 7 shades, from ivory to mahogany, and I really like the cream to powder finish. It's very flattering and works equally well on blemishes as well as circles and lasts a really, really long time.