This is one of the first things I ever sent to an e-mail list, which is now archived on the web (link):
The Gemara in K'suvos (60a) states that human blood is Biblically permitted. Rabbinically it is forbidden when it becomes detached from the body (eg if it drips onto food). The reason for this is either so that no one suspects you of eating forbidden animal blood (Rashi, Ran, Nimukei Yosef) or so that you do not accidentally eat animal blood instead of human blood (Rosh, Rambam). The Tur and Shulchan Aruch (YD 66) quote the first reason and the Aruch HaShulchan quotes the second. The Shulchan Aruch states that if blood drips into food it should be removed. If it has already been mixed in (and is less than 1/60 of the mixture, which is almost certain unless the blood really gushed out) then it is permissible to eat the mixture.
However, eating human flesh is a more controversial subject. There are at least four different opinions on the subject. The Ramban says that when the Gemara learns that eating blood is permissible, it also learns that eating human flesh is permissible. The Ra'avad (Hilchos Ma'achalos Assuros 3:4), the Rashba, and the Rosh (5:19) agree with the Ramban. However, the Rosh adds that the same rabbinical prohibition that applies to human blood (when it is detached) also applies to human flesh. The Reah and Ritva say that eating human flesh is forbidden because humans are not kosher animals and that the permission learned in the Gemara does not apply to flesh. The Rambam (Hilchos Ma'achalos Assuros 2:3,3:4) agrees that the permission does not apply to human flesh. However, he feels that the prohibition not to eat non-kosher animals does not apply to humans but the positive commandment to only eat kosher animals effectively excludes the option. The difference being that if you did eat human flesh and it was forbidden by a prohibition (Reah and Ritva) then you would be punished by a beis din (rabbinic court) with lashes. If it was only forbidden from a positive commandment (Rambam) then there would be no such punishment from a beis din. The Nimukei Yosef agrees with the Ramban that theoretically human flesh is permissible to eat. However, because it forbidden to eat flesh from a live animal (Eiver min hachai) and it is forbidden to derive any benefit from a human corpse, it is impossible in practice to use this permission. The Tur and Shulchan Aruch do not discuss this issue and the Rema (YD 79:1) says that human flesh is biblically forbidden (not like the Ramban, Rosh, etc.) to eat (not like the Ramban, Rosh, etc.) but does not specify which biblical prohibition.