Why are there so many First Congregational Churches in the US? A simple google earth search turns up hundreds of "First Congregational Churches" all over the U.S. What is the significance of this name?
Assuming you don't know about Congregationalism

Congregationalism is a system of governance where the local church independently and autonomously runs its own affairs. It does not specify any doctrine or hierarchy, which allows for the existence of 'Methodist Congregational' churches and 'Uniting Congregational' churches. There are two major denominations in the US which are Congregational throughout their structure - the 'National Association of Congregational Christian Churches' and the 'Conservative Congregational Christian Conference'.

Congregational churches are usually Protestant although there are also Congregationalist variants of other religious institutions.

As for the "first" bit

I haven't been able to track down any conclusive evidence.

Wikipaedia lists approximately 100 churches called "First Congregational Church" and only 2 called "Second Congregational Church". They are all in the US.