Reliable online sources for free knitting patterns include Ravelry, reputable yarn companies' websites like Lion Brand or Cascade, and blogs like Fiber Flux. Pinterest is also an exceptionally good source, but a free Pinterest account is required to access its content.
It is not necessary to have a Ravelry account to access free patterns on the site, but an account is required to save them online without downloading them. Knit Picks is another good, comprehensive site that offers both free patterns and patterns for sale, along with all required materials. Whether you're planning to knit your first hat or your thousandth hat, you can find a vast array of patterns online, available at every difficulty level.