My family has requested various felted gifts for the holidays. First up are felted slippers for my dad and one of my brothers. I used Cascade 220 yarn and did some felt swatching over Thanksgiving at my parents' because I have a front-loading washer.
Before felting, a 4 x 4 inch swatch knit on size 11 needles with two strands of yarn.

After felting (on left, a swatch with a single strand; on right, double stranded)

The unfelted slippers - about 16 inches long

And now for scale: an image of the unfelted slipper next to my foot!