For the clients that we represent who are purchasing a home we ask them if they would like us to throw them a Welcome to the Neighborhood "Block Party" as their closing gift. We send invites out to all of their neighbors announcing that our clients would love to meet their new neighbors, as well as sending invites to our client's guest list for their close friends/family. It doesn't take place of their housewarming but instead shows all of their friends and family where the home is and we get to meet all of them as well as any neighbors that show up. We then create some gift baskets (could get them donated from other sources) and have everyone sign a guest book (to be left for the buyers with their neighbors contact info on it) and do a raffle for the first RSVP, first person to the party, etc.
Of course, we provide simple food/drinks and have a couple of boxes we just keep in storage with our "party supplies" in them. We have a couple of carnival games for the kids, tablecloths, containers for food, etc. so it is very easy to coordinate. We also put flyers on the community mailboxes for added advertising and never let anyone inside the home (except the garage if it is inclement weather).
What it gives us is a list of contacts, interaction with people who know someone who is happy with our services, introduction to their possible referrals and a few photos of our clients in front of their new home. There is nobody in our area that does this and we have done a few and got good response, especially those that have kids that they wanted to meet the other kids in the area quickly.