Large open space, with set boundaries
One person is designated as "it" and begins a game of tag. One someone is tagged they become "it" as well and must become part of the blob by linking arms with the other people that are it. The blob continues to grow until it has swallowed all the participants.
Allow the blob to break into smaller groups once a certain number of participants have been assimilated. For example, when the blob has 10 people it is allowed to break into 2 groups of 5 which will then be able to break once they have 10 people. This encourages teamwork and strategy as the groups now work together to hunt the remaining free players.
If not careful the blob can become an unintentional game of "crack the whip" with people at the end of the blob being flung off. Encourage the blob to move at a speed safe for all participants. This can be fun to watch, a well thought out blob can slowly separate other players and surround them.