I really enjoy sketching interface concepts on the pads for sale at the bottom of this page on Tufte’s site. The pads are $10 for two small pads or one large pad.

The paper is the perfect weight, the off-white color reduces glare, and the very subtle grid lines help me maintain proportions. I prefer the 11” x 17” combination grid (grid at 1/4” and 1/2”) — the larger sized pad gives you plenty of space to isolate multiple sketches on a single page or draw a bunch of smaller screens to simulate a flow. Really nice product.