Sorry for the late reply, but I was asleep.

1. Custom ROMs: A nice way to extend your phone's lifespan, or just get a cleaner OS
2. Mifare Classic tool: I used that to clone my school lunch card for free
3. Minimum width settings: Maybe iPhones have this too, I am not sure. By increasing it from default 395dp to 705dp I have basically converted my phone to tablet. Apps consider tablet anything starting with 600dp.
4. LibreTorrent: Nice torrent client for Android, although you can't really seed much with mobile data since you're not connectable.
5. USB storage: I can connect a Flash drive or HDD to my phone. Cool.
6. Speaking of that, MLUSB mounter: Unfortunately paid and closed source, but mod APKs exist :) (unsafe). Since Linux kernel on Android is so limited, it brings support to extra file systems like NTFS and even UDF allowing you to use DVD or Blu-ray drive for browsing files on discs.
7. Speaking of USB, NokoPrint: Again, sadly closed-source app, but at least you don't have to unlock features. It allows me to print on my old HP printer directly from phone via USB with all the settings HPLIP provides.
8. Robot36: If I happen to hear SSTV transmission, I can use this nice decoder. It is open-source.
9. noaa-apt: This one isn't an app, but it can be compiled for ARM in Termux. It decodes APT transmissions from NOAA-15, 18 and 19 weather satellites they transmit in 137MHz band.
10. Termux: What I just mentioned above. And if I learn to configure it, Apache webserver, because why not.
11. Simple HTTP server: A slow but simple HTTP server app. Useful from sharing files to a full website on phone.
12. Matsuri: A really awesome proxy client
13. Android proxy server: Again, this could probably be done in Termux, but there's an app for it. Useful for sharing a VPN connection with other computers. Or setup TCP relay to Orbot. Or both. Or maybe using your phone's internet connection on school network with school computers. It allows for basic username password authentication.
14. F-Droid: More FOSS apps
15. USB Camera: Chinese app with ads that made me start using NextDNS. Allows me to comnect USB Composite capture card to phone and play NES games. Or a HDMI capture card from laptop. Or an actual camera. It can also run a server with the video feed. Easily allows changing between all supported resolutions and formats.
16. Screen stream: How about screencasting over HTTP? Sadly it doesn't support audio.
17. Limbo x86 PC emulator: Slow but fun to play with. Why not try Windows 98 on my phone, right? Even Windows 7 somewhat ran on it.
18. J2ME Loader: Let's replay those old Java games I played on my Sony Ericsson.
19. SDR++: Remember noaa-apt? How about recording it on your phone? A full on SDR (software defined radio) desktop experience. Although support for devices is limited on Android. Airspy, HackRF, RTL-SDR. I use it with RTL-SDRv3. The cheapest and most common SDR.
20. SatDump: A universal satellite decoder to use with your SDR. There's also Meteor in 137MHz band transmitting digital imagery. And if you get a satellite dish and L-Band helical feed (with correct polarization - remember dish is a mirror, it reverses it) there's a lot better imagery to get and additional satellites: Metop-B and Metop-C. But RTL-SDR isn't optimal for these higher bandwidths anymore. Consider something like AirSpy Mini.
21. SDRAngel: SDR app that tries to do everything. Unfortunately, it doesn't have scaling settings, so you'll have a hard time using it on phone.
22. Welle.io: Does your country have DAB/DAB+ radio broadcast, but you don't have a receiver? Well say no more! It is time to use your RTL-SDR for that. If you got the generic one (like $10) it saves you money that you would otherwise pay for a standalone receiver. The cost returns!
23. dump1090: An ADS-B receiver. Have you ever checked FlightAware or ADS-B exchange? How about getting that information yourself, directly from airplanes? Well you can do that with planes that are in sight.

Maybe I could find some more stuff, this is what I'd miss the most.