Gboard [or similar editing]
When editing something on a small screen, trying to get the cursor in the precise position with finger taps can be frustrating.
You tap the screen near the mistake, but the cursor lands just before or just after the letter you want to change.
If you use Gboard (and some other keyboard apps), you can use the spacebar as a trackpad.
Simply slide your finger left or right across the spacebar and watch as the text cursor moves smoothly and accurately through your text. You’ll wonder how you ever lived without it.
Chrome [and other browsers] tab selection
This one makes using multiple tabs in the browser much easier. Instead of tapping the little square tab counter icon and then scrolling through a stack of thumbnails, you can instantly jump between your open tabs.
Just swipe horizontally across the address bar. A quick swipe left or right will take you to the previous or next tab in your list. This gesture works perfectly in both Chrome and Firefox on Android.