Does the pushpin not describe your custom post type? Here are hundreds more icons to choose from. Icons are the GPL-compatible Font Awesome Icons.
This is the same icon features that comes with SuperCPT 0.2+, but pulled out of the plugin for those crazy birds who don’t use it.
This adds a function you can use, pti_set_post_type_icon( $post_type, $icon ); to set your post type’s icon. For instance, pti_set_post_type_icon( 'event', 'calendar' );. You can also set the global variable $pti_icons as an array of post type => icon names. Using the global variable is safer, since it doesn’t rely on a function (you could of course call if ( function_exists( 'pti_set_post_type_icons' ) ) to be equally safe using the function).
Screenshots
An example of a couple of icons next to the "Post" icon in a normal state
An example of a couple of icons next to the "Post" icon in a hover/active state