Audiowalks
Teri Rueb (GPS triggered) http://www.terirueb.net/i_index.html
Video walks
Janet Cardiff (also the Video-Walks!) http://www.cardiffmiller.com/artworks/walks/index.html
http://www.jena1806.de documentary: http://www.youtube.com/watch?v=AKXXkxD3ue4
Karl Heinz Stenz und Winfried Wrede: http://www.theaterwrede.de/dedv.htm
commercial
a company specializing on audiowalks: http://www.soundwalk.com