Many of Philadelphia's historical attractions are free, including Independence Hall, the Liberty Bell, Christ Church, Elfreth's Alley, Franklin Court, and the Edgar Allen Poe House.  The U.S. Mint is also free.

Other free activities include:

First Sunday of the Month and Wednesdays after 5pm: Pay as you Wish at the Philadelphia Museum of Art

Sundays Only: During summer, The National Liberty Museum is offering free  Sunday admission.  This small museum, located near Independence Hall,  celebrates the concepts of freedom, diversity and nonviolence, and displays some interesting glass sculptures.

Free Self-Guided Walking Tours

Free Outdoor Sculpture Tour

Free Pod-Cast Tours

Free Festivals at Penns Landing  http://www.delawareriverwaterfrontcor...

Visit the Race Street Pier, also on the Delaware, for a unique perspective:

You can also take a free Ranger tour  of various sites in Indepdendence Mall area with the National Park Rangers 

Or a variety of tours through the city with Free Tours By Foot,

First Fridays  Every First Friday of the month from 5-9pm, it's free to roam the many art galleries in Olde City is a fun way to hang out with friends and socialize, and the best part , its all within walking distance to many restaurants and bars to relax after roaming the galleries.

The Settlement Music School 416 Queen Street in Philadelphia has a great calendar of free music concerts.  What's terrific about the school, too, is the thousands of students who are assisted financially by the school.  The best and brightest of the young musicians can find their way to the Philadelphia Orchestra by sheer talent alone.  This is worth a listen to the professionals who play at the school, for free, as a donation to the community. Concerts include events such as "An Intimate Evening of Music for Cello and Piano," as a typical type of event. 

Visit Reading Terminal Market or the Italian Market  or one of the many farmer's markets.  The Sunday market at Headhouse Square is a summer destination

Take a walk along Boat House Row on the banks of the Schuylkill River, or attend one of the many regattas on the river

Also on the Schuylkill is the Fairmount Water Works Interpretive Center, an interactive museum that explains all aspects of water in Philadelphia:

"The Comcast Experience" Video Wall -- 1701 JFK Boulevard :  A spectacular blending of art and technology, The Comcast Experience is the world's largest 4 mm LED display and is located in the Comcast Center lobby. Viewers find themselves exposed to (and immersed in) a vast array of incredibly realistic imagery – from spectacular nature to compelling artistry, from nostalgic American culture to historic Philadelphia, from the celebration of sport to the celebration of cinema, all creating a personal experience. 

And, from Thanksgiving Day through New Year's Day at the top of the hour every hour (10 AM to 8 PM; excluding 5 PM on weekdays), "The Comcast Holiday Spectacular" is shown (duration: about 15 minutes).  It is full of captivating original holiday music, imagery, and more.