Are you using a different Ruby framework or need to access server data? If not, you can generate HTML from erb with the command line, then simply open the HTML:
<% # page.erb %>
<html>
<head>
<title>My erb</title>
</head>
<body>
<h1>it is: <%= Time.now %></h1>
</body>
</html>
To compile and open from the command line:
$ erb page.erb > page.html
$ open page.html