Basecamp does not have a means of listing links to external content. It does allow for uploading and so you can use this facility to have links to external content. I needed this for a small project that uses Basecamp to coordinate activities and Google Docs to hold content. The "link" to external content is encoded in an HTML document. For example,
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>TITLE</title>
<script>
var url = "URL";
</script>
</head>
<body onload="window.location.href=url">
<p>Automatically redirecting the browser to <script>document.write("<a href=\""+url+"\">"+xml_encode(url)+"</a>")</script>.</p>
</body>
</html>
Replace TITLE with the title of the content and URL with the URL to the content. (Don't forget to XML encode any HTML entities in the title and JavaScript escape any special characters in the URL.) Save this document to a file and then upload this file to Basecamp. When a user selects the uploaded file in Basecamp the user's browser will be redirected to the content at the URL. A lighter weight version of the HTML content is simply
<body onload="window.location.href='URL'"/>
