This site offers patterns for the actual original historical American Girl Dolls (Molly, Samantha, Kirsten) Molly is my favorite and I love her patterns. The pictures below are of things I created using those patterns.
I made this hat using the pattern for Molly's rain hat
This Christmas dress was made using the pattern for Molly's pinafore
This one I made also using the pinafore pattern
Here are some other sites that offer free sewing patterns for 18 inch dolls:
Liberty Jane This site sells patterns but they have a few that they offer for free, and two others if you subscribe to their newsletter.
Emily Rose They offer a pattern club and when you sign up they send you a coupon for a free pattern with the purchase of another pattern
Infinate Freedom I only found one actual pattern on this site, for a dropped wasit dress, but they have many pictures to show you how many different ways you can make it look
Doll Shoe This site offers a pattern for making doll shoes
Sewing.org has a free pattern for a pretty dress.
Trillium Design has a doll T-shirt tutorial including a T-Shirt pattern. This is made from knit fabric. Good way to upcycle t-shirts your kids have outgrown.
New Green Mama has another T-shirt tutorial and another T-shirt pattern. This is also made from knit fabric and it does not have separate sleeves so its easier for beginner sewers.
YouCanMakeThis.com has several free downloads for 18-inch dolls. You have to sign up to gain access to the downloads but that is super easy.
These are just a few to get you started, I'm sure if you dig deeper you will be able to find some more.