I've talked before about my oily skin. It's frustrating, and causes concealer to crease and all sorts of fun things...but I've found a wonderful product that helps control this issue.
The Laura Mercier Invisible Loose Setting Powder is fantastic. Where other powders can tend to look like you blobbed flour on your face, this setting powder is so finely milled that it is virtually invisible.
After applying a cream concealer under my eyes I set with this, and get zero creases throughout the day. Amazing (for me).
I couldn't go without this setting powder in the summer, or technically ever because my skin doesn't just show off its oily glow in the summer. It's pretty much year round.