Secret bars in Philly?

I've only been to the Pen & Pencil Club (http://www.penandpencil.org/). I knew a member and got in that way. I believe you need a keycode combination otherwise. Maybe things have changed. During the early evening it's a professional crowd. A rougher one shows up in the early morning hours. In any event, check out this link: http://www.thrillist.com/drink/philadelphia/secret-bars-in-philly. It includes the Pen & Pencil club. Guess they're not so secret anymore.

Hop Sing Laundromat on Race between 10th and 11th is great. 

Cool article that Greg N posted, yep those aren't really secret but still great in their own right and keeps out random people walking in. Fiume is one of my favorites in West Philly.

As far as actual after hours bars/clubs yes they exist in Center City and industry workers are your best bet in. Make friends with a bartender (offering them drinks is timeless) and ask where they like to go drink after they get off work.