Some defenders of the "Plan B" birth control pill claim that it is not an abortifacient.

It is. From Drugs.com:

Ethinyl estradiol and levonorgestrel are used together in this product as an emergency contraceptive (EC) to prevent pregnancy after contraceptive failure or unprotected intercourse. Ethinyl estradiol and levonorgestrel prevent ovulation (the release of an egg from an ovary), disrupt fertilization (joining of the egg and sperm), and inhibit implantation (attachment of a fertilized egg to the uterus).

Source: http://www.drugs.com/mtm/preven-ec.html

Even the makers of Plan B admit it can prevent implantation:

It is possible that Plan B One-Step® may also work by preventing fertilization of an egg (the uniting of the sperm with the egg) or by preventing attachment (implantation) to the uterus (womb).

Source: http://www.planbonestep.com/faqs.aspx