This dead-simple extension allows you to block distracting websites for a period of time your specify. You get to decide which websites to whitelist and which ones to blacklist. Block & Focus comes with statistics, keyboard shortcuts, breaks, and a few other functionalities but is overall a really simple and effective tool.
Limit allows you to put time limits on distracting websites. You get to choose how many minutes a day you're allowed to spend on each. After that, the website is blocked. It's a simple and effective tool for avoiding distractions and rabbit holes.
Momentum replaces your default new tab screen with a beautiful, personalized dashboard. It shows you the time, current weather, and a beautiful landscape photo. It comes with simple productivity tools such as a pomodoro timer, to-do lists, focus time, link lists etc. Even if you don't use the productivity tools, it's just gorgeous to look at.