http://paul.lost-thoughts.net/entry.html
Code:
<html>
<head>
</head>
<body><form name="form1">
<select name="compability" onChange="window.open(this.options[this.selectedIndex].value)">
<option value="http://yourwebsite.com/mac.1024x768.300k.html">Macintosh: 1024x768: 56k</option>
<option value="http://yourwebsite.com/mac.1024x768.56k.html">Macintosh: 1024x768: 56k</option>
<option value="http://yourwebsite.com/mac.800x600.300k.html">Macintosh: 800x600: 300k</option>
<option value="http://yourwebsite.com/mac.1024x768.56k.html">Macintosh: 800x600: 56k</option>
<option value="http://yourwebsite.com/mac.1024x768.300k.html">Windows: 1024x768: 300k</option>
<option value="http://yourwebsite.com/mac.1024x768.56k.html">Windows: 1024x768: 56k</option>
<option value="http://yourwebsite.com/mac.800x600.300k.html">Windows: 800x600: 300k</option>
<option value="http://yourwebsite.com/mac.1024x768.56k.html">Windows: 800x600: 56k</option>
</select>
</body>
</form></html>
A rather blunt coding of it. I also added a basic OS option, but this could be replaced with IE & Firefox, seeing as they seem to be the most popular browsers at the moment.
You could also keep your website as image-free as possible, and this would make the need of having internet connection speed choice uneccessary. (It would load relatively quickly in both).
Going back to operating systems, making sure it's compatible in Internet Explorer, Firefox (and maybe Safari) in the one go would get rid of the option of OS, thus making the list neater and smaller, so it'd really be just monitor resolution.
Code:
<option value="http://yourwebsite.com/1024x768.html">1024x768</option>
<option value="http://yourwebsite.com/800x600.html">800x600</option>