paddingTopLeft - Reserve space on the map for content

Center the map, allowing for an overlay
Content
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>paddingTopLeft - Reserve space on the map for content</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>
<style>
#content {
    position:absolute;
    top:0;
    left:0;
    width:200px;
    bottom:0;
    background:#000;
    background:rgba(0, 0, 0, 0.5);
}
</style>

<div id='map'></div>
<div id='content'>Content</div>
<script>
var map = L.mapbox.map('map', 'examples.map-9ijuk24y');

map.fitBounds([
    [39.0, -77.18],
    [38.72, -76.89]
], {
    // this preserves the space from the left of the real map to 200px
    // for content. the format is [x, y]. See
    // http://leafletjs.com/reference.html#map-paddingtopleft
    // for full documentation and other options
    paddingTopLeft: [200, 0]
});
</script>
The code and documentation to mapbox.js is hosted on GitHub where you can contribute changes and improvements.