These are mostly for knitting but they have some good general 18" doll projects as well.
This is a general free 18 inch doll dress pattern that could be modified in a number of ways with fabric and adornment choices.
These Doll Clothing blueprints aren't free but there is a large variety so we decided to include it here.
These aren't free, although $8.85 is a steal. The best part about these 18 inch doll dress patterns is that they included an option for a matching little girl dress. This site is great as it has lots of tips and tools to help and encourage your children to sew.
Free crochet clothing patterns for American Girl 18 IN dolls. We like this one because we love the little scarves and blankets. A good introduction to crocheting for anyone.
American Girl and 18" patterns. Not every doll clothes pattern is free - but a great listing of cheap and low cost 18" doll patterns from across the web.