Her most famous work is The Book of the City of the Ladies. In this work, the narrator, despairing of having been born a woman, is visted by three women: Lady Reason, Lady Rectitude, and Lady Justice, who promise to build a city for good women, a refuge adequate for keeping their despisers at bay:
There is another greater and even more special reason for our coming which you will learn from our speeches: in fact we have come to vanquish from the world the same error into which you had fallen, so that from now on, ladies and all valiant women may have a refuge and defense against the various assailants, those ladies who have been abandoned for so long, exposed like a field without a surrounding hedge, without finding a champion to afford them an adequate defense, notwithstanding those noble men who are required by order of law to protect them, who by negligence and apathy have allowed them to be mistreated. It is no wonder then that their jealous enemies, those outrageous villains who have assailed them with various weapons, have been victorious in a war in which women have had no defense. Where is there a city so strong which could not be taken immediately if no resistance were forthcoming, or the law case, no matter how unjust, which was not won through the obstinance of someone pleading without opposition? And the simple, noble ladies, following the example of suffering god commands, have cheerfully suffered the great attacks which, both in the spoken and the written word, have been wrongfully and sinfully perpetrated against women by men who all the while appealed to God for the right to do so. Now it is time for their just cause to be taken from Pharaoh's hands, and for this reason, we three ladies who you see here, moved by pity, have come to you to announce a particular edifice built like a city wall, strongly constructed and well founded, which has been predestined and established by our aid and counsel for you to build, where no one will reside except all ladies of fame and women worthy of praise, for the walls of the city will be closed to those women who lack virtue.[City of the Ladies 1.3.3]
I haven't been able to find a copy online; but you can read a summary of it. Christine also wrote The Treasure of the City of the Ladies, which discusses the proper education of a woman.
She became involved in a literary dispute over the Romance of the Rose, which she regarded with some detestation as immoral and misogynistic (see part of her reasoning here).
She was one of the first French writers to celebrate Joan of Arc's restoration of the fortunes of the Dauphin in 1429; you can find an English translation of her beautiful Le Ditie de Jehanne d'Arc online (see also here for a prose translation); it was written within two weeks of the Dauphin being crowned king.
You can read English translations from several of Christine's works (see also here for a good list); there's also an online concordance to her works (in test phase) at the Christine de Pizan Database.
