I honestly can't remember whether this little brush costs €1.50 or €2.50, but either way, it's less than my bus fare home, and you need to try it.
I think this is labelled in-store as a 'contour brush' or something to that effect but I've actually never used it for that purpose. The synthetic bristles, which are packed neither too loosely or too tight, make the perfect companion for liquid foundation or concealer. It just blends everything in seamlessly - I particularly love using it with my favourite summer foundation, Shiseido UV Protective Liquid Foundation, as well as an old reliable concealer, Collection Lasting Perfection. It's great for applying blush, as well. I've had two of these on the go for the last while and have just picked up a third!
Disclaimer: I received my first one of these brushes at a Penneys press day months back, and picked up the rest myself.