From Tosfos in Niddah 13a it would seem like women are permitted to masturbate. However, I haven't seen any modern teshuves permitting or prohibiting it. According to Wikipedia,
The Halakhic attitude towards a female masturbation is found in the opinions of the Ben Ish Chai, who says that it is wrong because it creates evil forces (Kelipos), and Rabbi Moshe Feinstein, who forbids it because it involves indulging in sexual fantasy about men, which falls under the prohibition of forbidden thoughts, which are forbidden for women as well. However, it does not carry the severity of male masturbation, because it does not involve the release of seed.
Is it true that R' Moshe prohibited it? Are there any other modern Poskim that ruled on its permissibility?