Pythonista lets you use your pocket UNIX workstation as a workstation. I use Pythonista, if not every day, very heavily on the days I use it. As always it's crippling of Apple that there's no upgrade pricing, so I can't give him more money every year that I keep using it. The new keyboard module is an interesting script launcher, but I already wrap a bunch of utilities in a main menu program.
- Panic's Coda and Coda for iOS (née "Code Editor" WTF) is the only other one that's really functional; I've built real web sites out of it, but I mostly use it for ssh. Sweet baby Cthulhu, I hate Panic's crooked-text "designer" sites, I hit Reader view on those instantly. Designers shouldn't be allowed access to CSS or JS.
- Hotpaw BASIC still works (as does his Chipmunk BASIC on the Mac), but hasn't been updated in 2 years. Not that I want to program in BASIC, but it's better than no programming at all.
- The iPad used to have a very nice "BASIC!" (with a structured BASIC and a bunch of system functionality), and a very limited "iSkeme" (scheme interpreter, R5RS-ish? with nothing but text I/O), but they were killed in the 64-bit-pocalypse. Update 2020-09: miSoft Basic! has been updated. Searching for this is utterly impossible!
- Workflow (née Apple Shortcuts) is great for putting a few tasks in a row but you'd go insane trying to write anything complex from drag-and-drop clicky boxes.
- Apple's Swift Playgrounds on iPad is a tutorial, not really usable for applications AIUI.
- There's a bunch of "kids learn to code!" apps that are mostly ripoffs charging $60/year to play robot tanks. Do not buy anything like this.
I dunno if the 'droids have anything comparable, I'm sure they can root their phone and try to use vi in a busybox shell, but that's not a reasonable work environment for a thumb-sized on-screen keyboard.