I am in Philadelphia for a few days for only the second time in over a decade. Since New York is only two hours away I should get there more often. There are numerous ways to make the trip through New Jersey in addition to the good old $15 intercity bus trip. Philly can be a transit fan's paradise, since its transit agency, SEPTA, operates one of the most diverse transit systems in North America: buses, trolley buses, streetcars, light rail, subway and elevated heavy rail, and regional rail. Only Boston's MBTA has a more diverse system as it also operates a handful of low-ridership ferries. San Francisco joins the list if you include the entire region (not just a single transit agency), but MBTA and SEPTA serve their entire regions.
Expanding beyond SEPTA, transit services in the Philadelphia region include the PATCO High Speed Line, a single heavy rail line to neighboring Camden and a few NJ suburbs; NJTRANSIT rail to Atlantic City; and many NJTRANSIT local and express buses feeding the transit hub that is Center City. Across the river in Camden, NJTRANSIT runs the River Line light rail to Trenton, and the SEPTA Regional Rail connects directly with both NJTRANSIT rail to New York City and Delaware's DART First State bus system, as well as a handful of smaller local transit operators in adjacent counties.
As an aside, both SEPTA and DART First State have buses on loan to NJTRANSIT to fill in for buses damaged during Hurricane Sandy.