Have seen loads of tutorials for photoshop how to make a letterpress effect as seen above but non of them for GIMP (http://gimp.org). It is a trendy effect these times on the web and yes, it is fairly easy to create one.
My blog-post with calendar icons is nearly one year old. I've seen quite a lot of people using it and was surprised. One of the best upgrades was by Joost de Valk (Yoast.com). His article on CSS-Tricks shows his idea to place a bubble with comment count over the calendar icon. Read how it works and how to implement it in your drupal theme.