Frailing banjo is probably the easiest thing to learn. Learn one picking pattern and a few chords and you can play and sing thousands and thousands of folk songs.

Here are some free resources to check out:
Old Time Banjo video workshop
Frailing Banjo video workshop
http://www.ezfolk.com/banjo/howtao/
http://tangiersound.files.wordpress.com/2007/06/five-strings.pdf
The Outlaws and Scalawags Songbook
The Daily Frail archives
Folk Song of the Day archives
Four text workshops on frailing

-Patrick