One thing that I usually need is to take a screenshot. It can be:
Sharing a UI update for my pull request
Showing a UI bug on my bug report
Creating a screenshot for my presentation
Creating a screenshot for documentation, tutorial, or notes
On Windows, I found the Snipping Tool is handy for this. But I don't have the Snipping Tool on Ubuntu. Here I found this powerful but simple-to-use screenshot tool, Flameshot.
You can install it not only on Linux but also on Windows or macOS. Check here for more information.
The features that I like:
Select the area of your screen easily. You can also adjust the area before finalizing it.
You can add a delay. It's useful for example taking a screenshot on the context menu or hovering over a button.
You can copy it to the clipboard. No need to save it and populate your disk. Of course, you can save it also. My usual case is to take a screenshot, copy it, and paste it somewhere I need it.
You can edit it: add lines, arrows, rectangles, blur, text, markers, and some others. You can set the color (using right click) and set the size (using the mouse wheel) also.
Undo and Redo. It's handy to have these features when you are editing
There may be more features that I don't know about/use (e.g. upload it online platform). But those are more than enough for me. Those features are also easy to use. The trickiest one probably taking a screenshot with delay. You need to open the launcher, set the delay, and take the screenshot. For a normal screenshot, you can just launch it from the tray.
There is also a configuration, but using the default is already good.
Another good thing, it's open-source and free!
So, if you need a powerful yet simple-to-use screenshot tool, I can recommend Flameshot.
Notes: all images are taken from the Flameshot website. Visit it!
PS: Previously, I use Shutter. But for a reason I can't remember, I was forced to find another tool. It's also a good tool.