The one that I got fairly recently and have really come to appreciate is my RT stippling brush. I use it for cream and powder blushes and it feels like I can be so much more precise, both in building and in placement, than with a regular blush brush.
It's especially good for really pigmented cream blush. I would be hard-pressed to do a good job applying my Nyx Glow cream blush, for instance, without my stippling blush.