It is possible to purchase with less than 20% down. On the government loan side, VA and FHA will permit 0% and 3.5% down payments, respectively.
On the "conforming" side, you can put down as little as 3%, with a loan amount up to $417,000.
Usually, if you put down less than 20%, however, you will pay a type of mortgage insurance, or you will need a second mortgage to facilitate the purchase. I cover all of this in a free monthly webinar that I host and the next one is on Oct. 8. If you'd like to join us, the registration link is below.