Several people have posted about this before, but I thought I'd just reiterate what others have said for the sake of completeness. Goo conducted a poll on Japanese celebrities who could best be classified as mori girls, and three of my favourite Japanese actresses made it into the top three!
#1: Almost half the voters selected Yu Aoi (蒼井優) as the #1 mori girl. She is undeniably the face of the mori girl trend. I've loved Yu ever since I saw her in All About Lily Chou-Chou, and she just keeps growing as an actress. In an industry where girls struggle to outdo each other in terms of cuteness, Yu Aoi is refreshingly earthy and natural. She's got an indie sensibility that is reflected in her unique style and image. Her most typically mori girl role has been Hagumi Hanamoto in Honey and Clover, but really, in most of her films she portrays individualistic, quirky girls.
#2: The adorable Aoi Miyazaki (宮崎あおい). This girl has the cutest face I've ever seen. You may have seen her as the irrepressible Hachi in Nana. While I wouldn't say she has as mori girl a look as Yu Aoi, she is often photographed in cute, loose, layered outfits. Her role as a quirky photographer in Heavenly Forest has been identified as a mori girl icon.
#3: One of the prettiest young Japanese actresses in my opinion, Juri Ueno (上野樹里) is famous for her scatterbrained, free-spirited character in the Japanese drama Nodame Cantabile.
#4: I admit I don't know much about singer and host Kaela Kimura (木村カエラ). She looks spunky and unique from the pictures, though.
#5: Yuki Isoya (磯谷有希) is a prominent Japanese singer who has been part of several bands. She's got a fun image which belies her age--she was born in 1972. That goes to show that the mori girl subculture isn't restricted to adolescent girls!
#6: Hiromi Nagasaku (永作博美)
#7: Eri Fukatsu (深津絵里)
#9: Aiko Yanai (柳井愛子)
#11: Chiaki Ishikawa (石川智晶)
#12: Kumiko Aso (麻生久美)
#14: Rie Miyazawa (宮沢りえ)
#15: Mikako Ichikawa (市川実和子)
#16: Tomoko Kawase (川瀬智子)
#16: Miho Tanaka (田中美保)
#16: Ikuko Harada (原田郁子)
#19: Yuko Ando (藤裕子)