Knitting on the Net, Straw.com and Simplicity.com have slipper sock patterns available for free download. Slipper sock patterns can also be found on the Lion Brand Yarn website, however a free account is required in order to view them. Ravelry.com has a slipper sock pattern available as a paid download.
Knitting on the Net's pattern is for a simple style of slipper sock that features a novelty trim around the top of the heel. This pattern is closer to a traditional sock style and features a different color on the toe and heel.
Straw.com's free pattern is also for a simple style slipper sock, however theirs is lightly felted for a different look. Once the knitting is completed, the slipper socks should be washed in a warm wash along with jeans or towels to create the felting effect. These slipper socks are made using the twined knitting technique with Taos yarn.
Simplicity.com's pattern is for a Nordic style slipper sock that features stripes and a flower pattern. This is a more complex design that requires four colors of yarn.
For a more advanced knitter, the pattern for a knitted Converse Chuck Taylor-style slipper sock is available to purchase from Ravelry.com. This is a unisex-style sock that the creator rates as medium difficulty. The pattern is available in U.S. sizes 6 to 11.