There's also

Brandywine Friends of Old Time Music
http://www.brandywinefriends.org/

Philadelphia Folksong Society
http://www.pfs.org/PFS.php

Concerts at Longwood Gardens (nearby Longwood PA)
http://www.longwoodgardens.org/PerformingArts/performartshome.htm#For daily listings

Old Time Jam almost every Friday Night at the home of Bill & Fritzie Wisdom, Bryn Mawr PA
Summer is the old time festival season so call 610-527-7080 to verify that they're home.

Brandywine Dulcimer Fellowship (Wilmington DE) meets 1st Friday of every month beginning at 7:30 PM at Grace Lutheran Church
The name says it all: "Dulcimer".
Call Bob & Bettie Scott at 302-762-1422 for directions if interested.