Proxy bid is an auction technique in which the winning bidder pays the amount the second bidder is offering increased by the bidding increment. This has an advantage that the final offer is consolidated, since more bidders are willing to pay the price around the final price. Also, it has the advantage that a bidder can set the highest offer in advance and then the bidding increases automatically in case other bidder places an offer. It has its drawback as well: the auction site or the seller might try to push up the price illegally by making offer just below your maximum bid (shill bidding).
Almost all domain auction sites support proxy bidding, namely GoDaddy auctions, Snapnames, Namejet, Dropcatch etc.