Aside from various neighborhoods in West Philadelphia, Montgomery County's Main Line is probably best considered Philadelphia's "mansion district."
My favorite is in Gladwynne. It's called Woodmont and it's home to "Mother Divine," leader of the International Peace Movement Mission, a dwindling cult. The house is freakin' amazing...
The story is even more amazing, a long read but well worth it...
http://www.mainlinetoday.com/Main-Li.../Prodigal-Son/
Ardrossan Estate in Villanova was part of the inspiration for the movie
Philadelphia Story...
La Ronda is one of the many Main Line mansions demolished for something more modern...
Aside from architecture buffs, I don't think a lot of people know that Philadelphia has Newport, RI in its back yard, and a lot of the houses are threatened.
Bloomfield Mansion in Villanova was destroyed by a fire, but the owners are rebuilding it...so it's not all bad...