It depends on what kind of party and what your overall goal is.
If you want family friends so that your families can get together, then a cookout or BBQ, a football tailgaiting party or something like that is a really good option. You want to have activities for the kids to do while the adults are watching the game, or playing bunko, or whatever. A relaxed and layed back atmosphere is the best. Do it in the afternoon, after nap, before dinner.
If you want to make friends for your kids, primarily, then a more child themed party is in order. Still do a cookout or whatever so you can meet the parents but have the activities geared more toward the children with things like, an obstacle course (homemade), a bounce house, a dance party using the wii or xbox games, a costume party using this years halloween costumes. Don't make it a birthday party or a party where people have to buy and bring a gift, just say it a fall party, kid olympics, or whatever and sell it to friends and neighbors as an opportunity for the kids and adults to all get to know each other at once in a safe and friendly way. Do it at the same time as the cookout.
If it's for the neighborhood, so you can get to know your neighbors, initiate a block party, with a potluck dinner, music, and so on. Do it at around 5 PM, time for dinner but still enough time to visit and get home early.
If it's for the adults, so you can make some adult friends, invite people to a poker or game night. You can play spades, dominoes, hearts, texas hold 'em, and do it tournament style, with the winner or winning team to receive a prize. Do it in the evening, around 7 PM, after dinner and time spent with kids and close enough to bedtime so parents don't feel like they are giving up family time.