(Updated December 21, '09)
If you are interested in studying more in detail about the differences between Judaism and Christianity, choose from the lists below. It is not organized in any particular order.
Jews For Judaism an international anti-missionary organization. An extensive website.
Their Hollow Inheritance (A complete book for free). A great book, I recommend it. Among other things, this book offers a great deal of evidence for some rather surprising information about the origins of christianity.
Outreach Judaism bringing back Jews from the Church
Torah Life And Living: The Emergency Room for Jewish Souls.
A site by professionals dedicated to anti-cult work and anti-missionary work. They provide help if a family member has been seduced by a cult, and Exit Counseling for those who themselves are or have been in cults.
Shomrai HaBrit-Keepers of the Covenant
Project Truth: Response to Christian missionaries.
The Jewish Home: Counter Missionary Articles and Lessons
Some articles at Aish HaTorah about this subject.
Anti-Missionary Study Hall
Why don't Jews Believe in Jesus? and many other relevant articles.
Countermissionary.org In their own words: "....this site is dedicated to defeating the attempts of missionaries of all types to deceive Jews into converting to another religion."
Hareshima's Counter-Missionary Links Page.
If you have a good link to suggest, please email me.