It's ossur for a man to look at a woman for pleasure. Even to look at her small finger with the intention of having pleasure from it (See EH"E Siman 21 Sif 1.)
Is there any prohibition for a woman to look at a man for pleasure?
There are two opinions in the medrash cited in Aryeh's answer; R' Yehuda permits women gazing upon men. His opinion is accepted by Sh'muel in B'rachos 48b. Although R' Yochanan follows R' Yosi's interpretation of the pasuk there, the gemara does not record that he added the phrase אם כן עשית בנות ישראל כזונות. In fact, R' Yochanan himself allowed women to gaze upon his handsome countenance (B'rachos 20a, Bava M'tzi'a 84a).
Although the Sefer Chasidim implies that it would be forbidden for a woman to gaze at a man (§614, "והוא הדין לאשה שלא תשמע קול איש שמכל שהאיש מוזהר אשה מוזהרת"), this is not generally accepted as halacha. Both R' Moshe Feinstein (Igros Moshe EH vol. 1, §69) and R' Ovadia Yosef יבלחט"א (Yabia Omer OC vol. 1, 6:5) rule that women are permitted to stare at men for aesthetic pleasure as long as they do not fantasize about the man or stare licentiously.
As far as the Sefer HaChinuch quoted in Aryeh's answer, it prohibits women from fantasizing about men other then their husband, but it does not prohibit them from gazing upon other men generally speaking.
Although they're marginal, there are authorities who discuss this.
In Sefer HaChinuch, at the end of Mitzvah # 188 (קפח):
And in Yalkut Shimoni, Shmuel I, # 108 (קח):
So, there are clearly sources that forbid it. The bigger issue here is whether women have the same prohibition of hirhur that men have (which I think deserves its own question). If they don't, there shouldn't be an issue at looking at men - unless, of course, the act of looking at men might cause a stumbling block for men to transgress (but even that sounds tenuous). In any case, neither the Rambam nor the Shulchan Aruch discuss women forbidden to look at men as halacha.