Polylines

Adding lines and polygons to the map
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Polylines</title>
  <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no' />
  <script src='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.js'></script>
  <link href='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.css' rel='stylesheet' />
  <!--[if lte IE 8]>
    <link href='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.ie.css' rel='stylesheet'>
  <![endif]-->
  <style>
    body { margin:0; padding:0; }
    #map { position:absolute; top:0; bottom:0; width:100%; }
  </style>
</head>
<body>
<div id='map'></div>
<script>
var map = L.mapbox.map('map', 'examples.map-9ijuk24y')
    .setView([38.89399, -77.03659], 17);

// Define circle options
// http://leafletjs.com/reference.html#circle
var circle_options = {
    color: '#fff',      // Stroke color
    opacity: 1,         // Stroke opacity
    weight: 10,         // Stroke weight
    fillColor: '#000',  // Fill color
    fillOpacity: 0.6    // Fill opacity
};

var circle_one = L.circle([38.89415, -77.03738], 20, circle_options).addTo(map);
var circle_two = L.circle([38.89415, -77.03578], 20, circle_options).addTo(map);

// Create array of lat,lon points
var line_points = [
    [38.893596444352134, -77.0381498336792],
    [38.89337933372204, -77.03792452812195],
    [38.89316222242831, -77.03761339187622],
    [38.893028615148424, -77.03731298446655],
    [38.892920059048464, -77.03691601753235],
    [38.892903358095296, -77.03637957572937],
    [38.89301191422077, -77.03592896461487],
    [38.89316222242831, -77.03549981117249],
    [38.89340438498248, -77.03514575958252],
    [38.893596444352134, -77.0349633693695]
];

// Define polyline options
// http://leafletjs.com/reference.html#polyline
var polyline_options = {
    color: '#000'
};

// Defining a polygon here instead of a polyline will connect the
// endpoints and fill the path.
// http://leafletjs.com/reference.html#polygon
var polyline = L.polyline(line_points, polyline_options).addTo(map);
</script>
The code and documentation to mapbox.js is hosted on GitHub where you can contribute changes and improvements.