parkrun started out as a UK based organisation and has become pretty big in the last few years, very few towns of any size that don't have one. I've been a runner for a long time, but I love parkrun because it's got a great social element and genuinely is open to everyone. My local one regularly has people in their 80s going round (and once we had a lady on a zimmer frame doing 5k), but we've had an Olympic athlete occasionally. My kids love to do it too. Volunteering to marshall/man the timers etc. is also great fun and very sociable. It's spread from the UK to other European countries and Australia, South Africa etc.
Parkrun doesn't seem to have taken off in the USA nearly so quickly. I think part of it is that you were already pretty well served with running events and part that I think the concept of free/volunteer run events isn't so strong, mainly because of litigation/insurance concerns.
http://www.parkrun.us/ shows only 4 events running right now - and most of those don't look very big.