I just gave my son $20,000, how much gift tax do I owe?

From FinancialPlanning

Because $20,000 is greater than the current (2006) $12,000 limit on gifts to any one recipient in a single year, you have a reportable gift. You may not be liable for any tax, though. Each year you can make as many $12,000 gifts as you want, and only the ones greater than $12,000 are reportable (on IRS Form 706). But only when the sum of all your reportable gifts is greater than $1,000,000 do you pay gift tax. So very few people actually end up paying gift tax.

In this example you would have a reportable gift of $8,000 (the $20,000 gift minus the $12,000 annual exclusion), but if you had made no other reportable gifts before this, you wouldn't owe any gift tax.

Even if you don't pay gift taxes, reportable gifts do reduce the amount that you can pass to heirs free of estate tax. This becomes a concern only for individuals with larger estates, who frankly can afford paid help instead of using MIFP-FAQs!

Note on gifts by married couples - because any individual can gift $12,000 to any other individual, a married couple can gift $24,000 per year to any other individual. If the gift comes out of only one spouse's checking account, though, you need to file Form 706 and elect "gift splitting" which simply declares the gift as being from both of you.

For more information see IRS Publication 950, Introduction to Estate & Gift Taxes, as well as Form 706 and its instructions.