The Lily Auction--mostly Daylilies if you are not familiar with it--is one of those sites.
"In the Lily Auction, if a bid is made within the last 5 minutes, the auction will not close until 5 minutes after the last bid is made. This is to prevent snipers from taking the bid in the last second. All buyers will have at least 5 minutes from the time of the last bid made to decide if they would like to bid higher."