So many. Things, WaterMinder, Solitaire, Deliveries, 1Password, Newsify, PCalc, MiniWiki, Shazam. And of course plenty of built in apps: Now Playing & Music/Podcasts, Alarm/Timer, Calendar, Mail, Messages, Home, Maps/Find People, News, Phone, Sleep, Weather, Wallet, etc.
Any
#applewatch apps can’t live without? For me it’s been the Timer, Alarm, and BabyTracker — oh and podcast / audible controls