free web, http://www.etymonline.com/
free Windows app, 1888 OED, 15000 pages searchable: https://archive.org/details/oed11_201407 (modern CD version costs $200+ on Amazon)
$9 iOS app, English BigDict, https://itunes.apple.com/us/app/english-bigdict/id397603643?... (includes historical references to ancestral words in other languages)
$30 iOS app, Shorter Oxford English Dictionary, https://itunes.apple.com/us/app/shorter-oxford-english-dicti... (be careul, there are several similarly named apps made by licensees of the OED brand, this one has 600K words)