To set this up, Judah has had three sons, two of whom are Er and Onan. Judah has taken a wife for Er, but Yahweh didn't like Er, so he killed him, and...
Then Judah said to Onan, "Go in to your brother’s wife, and perform your duty as a husband’s brother to her, and raise up offspring for your brother." Onan knew that the offspring would not be his; so when he went in to his brother’s wife, he wasted his seed on the ground in order not to give offspring to his brother. But what he did was evil in the sight of Yahweh; and he killed him also. (Genesis 38:8-10)(EDIT 6-29-08)
The main reason I chose this passage is because it's humorous. It's humorous because (1) the way it is written/worded, and (2) God killed Onan over something so trivial.
I selected it for another reason, for which I am making this edit. This passage is where the concept of onanism comes from. This concept is where we get two of the most inane Church doctrines:
1. Masturbation is immoral / wicked / condemned by God.
2. Contraception is immoral / wicked / condemned by God.
Yes, indeed this is the only passage these are based on. The traditional concept of onanism referred to masturbation, but has changed to reflect more of contraception. In the case of the Catholic Church, this passage condemns both masturbation and contraception as immoral.
When additional passages are tried to be used in conjunction with this passage, that is when it gets really twisted. We come to Ray's favorite passage of Matthew 5:28 wherein Jesus (incorrectly) defines adultery as simply lusting after a woman. So, even if onanism isn't condemning masturbation, they hold that this is, as it is likely that a male who masturbates is lusting after a woman, and is thus committing adultery. However, since this is the vile concept of a thoughtcrime, this leaves open a rather bizarre loophole: you may masturbate as long as you do not lust after woman.