Here's a good game for a family reunion, neighborhood party, or any gathering that involves several families and lots of kids (plus an adult to supervise). Divide into two teams, evenly matched, and form facing lines. Flip a coin to see who goes first. Members of that team hold hands and call for someone on the other team to try to break through their line. Surely you remember the chant: "Red rover, red rover, send ___ right over!" The person called runs toward what he thinks is the weakest link in the chain (to keep things fair and safe, impose a rule that adults can only run at other adults -- no charging little kids). If he breaks through, he gets to pick anyone from the line to go across the yard and join his team. If he bounces off or ends up dangling in the air on two players' arm, he has to join the other team. The game goes on until everyone is on one team.
This is "tag" with more moving parts. When you're "it," the players you tag have to stay frozen wherever you tagged them. But other players can then tag them, which has the effect of "unfreezing" them until you get them again. The game ends if whoever is "it" manages the impressive feat of freezing everybody, or when any one player has been frozen three times (or five times for younger kids). At that point, he's "it" and the game resumes.
Steal the Bacon
An even number of players line up on opposing sides of the yard. In the middle, between the two teams, is the "bacon" -- it can be a ball, a glove, anything. Have each team take turns counting off from 1, so that every team member shares a number with someone on the opposing side. The umpire or "caller" -- that's you -- calls out a number, and the players on each side who have that number step out onto the field. Whoever grabs the bacon first has to try to bring it back to his side, while the other player on the field has the task of tagging the runner with the bacon. If the runner gets tagged, the bacon goes back and the umpire calls a new number. A runner cannot be tagged until she touches the bacon, so players will alternate between making mad dashes or hovering around the bacon and trying to fake an opponent out.
Hide and Seek
The original rules are simple enough, but just to refresh your memory -- one person is "it"; she covers her eyes at the designated "base" and counts to 100. Then she goes off looking for the others, who try to sneak or run back to base before "it" tags them. Whoever is tagged is then "it."
Here's a cool twist that makes for exponentially more frantic running, especially if you play the game in a big yard or park with lots of hiding places: Instead of using a fixed object or landmark, make the "base" a person, and make sure that person hides, too. He doesn't have to hide in one place, either -- he can move around as much as he wants during the game. Most players have no idea where "base" is at any given moment, and they'll have no choice but to keep running until they find him. "Base" also has the option of running away from the kids who are trying to reach safety.
Duck, Duck, Goose
Everyone sits in a circle as one person -- the "fox" -- walks around the outside of the group, tapping heads and saying "Duck...duck...duck..." until he finally decides to tap a "goose," who then jumps up and chases the fox around the circle. If the goose gets the fox before he reaches the spot the goose vacated, the fox has to try again. If the fox makes it around without being tagged, the goose takes his place.