It's this crazy origami of a knit piece. Knit the triangle, then pick up side stitches as you knit from the toe. The only seam is at the back of the heel.
This pattern is for knit slippers, but I can totally see using a wool sweater and cutting and serging slippers from it! Home Slippers - DIY. this is knitting, but I bet I could use this to crochet some