node.js test code demonstrating an http.Agent configured with maxSockets = 5 appearing to "block" after the 5th http request has been sent. If sockets were being reused in the agent, I would expect all 10 requests to complete.

http_test.js

var http = require('http');
var url = require('url');

var agent = new http.Agent({
	maxSockets: 5
});

var path = 'http://www.google.com'

var options = url.parse(path);
options.method = 'HEAD';
options.agent = agent;

console.log('URL Options:', options);

var result = 0;

for (var i=1; i <= 10; i++) {
	
	var req = http.request(options, function(res) {
		console.log(++result + ' statusCode: ' + res.statusCode);
	});
	
	req.end();
}