For Desktop/tablet:
Facebook, twitter, Netflix, Instagram, Aviary's "photo Editor", Messenger, Book Viewer, treesize, crystaldiskmark, flipboard, skype
For mobile:
Slimsocial, tubecast, book viewer, Netflix, office lens, runtastic, WhatsApp, authenticator, jack of tools pro, skype, timber, fantasia painter, lastpass, notify mi band, Pandora,
qr scanner, torrex, video 360
Games for mobile: asphalt airborn 8, leo's fortune, shadow fight 2, dues ex go, lara croft go, prune, dream machine, hitman go, machaniarium, tiny troopers, freeblade, the two halo games, relic run, poly and the marble maze, modern combat 4 and 5
For tablet, games: limbo (and for apps, same as desktop, but they are even more handy).
Side note: most of those games can be fun on a desktop too for casual gaming, and unfortunately for tablet, there's a handful of touch friendly games that are not in the store, specifically trine 2, shadowrun returns series, civilisation (everything after 5), calvino noir, the room series, legend of grimrock 1 and 2.
If you combine the above games, with the mobile games, you get a pretty great selection. But you have to get each on from the right place- for example, if you get leo's fortune from steam, its not touch friendly. Same with machinarium. But with trine 2, you specifically need the steam version. It's very frustrating, unless you have someone save you the headaches. I wrote this awhile back for the purpose:
http://tap-that.co.nz/blog/2017/03/22/25-best-touch-games-for-windows-tablets/