Quite a few XPN shows are syndicated to other networks. For example, World Cafe, Mountain Stage. Otherwise, over the air it looks like this:

Streamed Worldwide on XPN.org/listen
WXPN 88.5 FM in the greater Philadelphia/South Jersey area
104.9 FM in the Lehigh Valley
90.5 FM Worton/Baltimore
88.7 FM Lancaster, York
99.7 FM Harrisburg