In its most simple form, a 3rd place is a neighborhood hang out. In our small town research, we have found the 3rd place an invaluable forum for communities to meet up informally in a variety of social settings. There are many cool examples out there - coffee shops, bakeries, craft breweries are most typical - but we have even seen them in retails spaces!
This short 'Not Just Bikes' you tube video offers a compelling thesis on the community value of a 3rd Space.