A clever website lets you find subway lines that you've ridden on and create a little online "badge" with all of the subway system icons. (See example, below.)
Make your own at http://metro.b3co.com, an astonishingly easy-to-use site. Type in a city's name, and the icon for its metro system will pop up.
UPDATE: Sept. 2, We regret to say that our commenting system does not allow you to post html code with your badges. We are sorry that you cannot post the icons of the metros you've visited on this blog post. We regret any inconvenience.
Here's an example:
Got at b3co.com!
[found via the blog cjw]