Some tricks that I found for our lovely HTC One. Feel free to add more if you have discovered something.
1. Background in text message app
2. Switch toggles using two finger shortcut
3. Phonebook browse by letter shortcut
4. Secure box
5. Default homescreen
6. Task list shortcut
7. Google now shortcut
8. Dial *#*#3424#*#* for self diagnostic
9. Save memory by using USB OTG
10. Turn on multitouch
11. Dial *#*#4636#*#* to force 4G/LTE
12. Screenshot shortcut
13. Screen mirroring
14. Share files using NFC
15. Remote control
1. Background in text message app
2. Switch toggles using two finger shortcut
3. Phonebook browse by letter shortcut
4. Secure box
5. Default homescreen
6. Task list shortcut
7. Google now shortcut
8. Dial *#*#3424#*#* for self diagnostic
9. Save memory by using USB OTG
10. Turn on multitouch
11. Dial *#*#4636#*#* to force 4G/LTE
12. Screenshot shortcut
13. Screen mirroring
14. Share files using NFC
15. Remote control