One of the most popular extensions with over 2.8 million Atom installs, Emmet makes writing HTML quick and effortless. Emmet is essentially a toolkit of snippets, but dynamic and on steroids.
For the frontend web developers, working with colors can sometimes be hard. Find the perfect color can sometimes be a painstaking task. With Color Picker you won't need to leave your text editor to access all the colors of the world, wow.
This extension is my favorite and I use it obsessively even when I know it won't change anything. Prettier is an opinionated code formatter for multiple languages. Basically, it makes your ugly code with all of its unnecessary whitespace, missing characters, empty lines, etc and reformats it to a standard that has been accepted by many.
After installing Linter, I can't imagine a life without it. Linter will attempt to notify you of any errors or other bits of useful information while you're writing your code. At save, it will also tell you what went wrong and where within your text editor even if your programming language doesn't support such features. My favorite feature is when I have an assigned variable but is unused throughout my code.
Scroll Through Time lets you use your trackpad or click wheel to, well, Scroll Through Time™. My only gripe with this extension is that if my code bleeds off the window to the right, scrolling with two fingers won't take me to the end of the line, instead I end up scrolling through time.