I've been working on a ChatGPT Chrome extension for about a month now. I released the first version a few days after ChatGPT was first announced.
The extension adds some extra useful functionality to the ChatGPT interface. Some examples of those features are
User input history
All of the user's input will be saved and you can easily go through them by pressing the up/down arrow in the input box

Prompt Library
Users can share their interesting prompts with other users with a single click right inside ChatGPT. I've personally learned a lot about how to write better prompts by looking at other prompts

Export Chats
ChatGPT will soon be monetized and everyone has to pay to use it. Being able to export all your chats to prevent losing access to them is very helpful. You can export all your chats in with a single click. Export currently supports markdown, text, and JSON format.

Safe Mode
This is an experimental feature that allows you to simply turn off the ChatGPT content moderation feature. This combined with the use of DAN(Do Anything Now) prompts would allow you to generate almost anything

Copy text and Char/Word counter
Some simple features to make it easier to play around with ChatGPT

This is the daily impression chart for my extension

As you can see, the number of impressions was very low (<200/day) for the first ~20 days. I believe two things had the most effect on the impression starting to grow at a much higher rate after that:
Continuing to add features. On average, I released a new version of the extension every 2 days. Sometimes new features, and sometimes bug fixes and improvements. I joined any ChatGPT Reddit or Discord channel I found and looked for the most requested features and tried to implement as much as I could.
I joined the official ChatGPT Discord channel, and became an active member. I'd answer other people's questions, and try to help them as much as possible. I spend most of my free time in the chatgpt-suggestions channel and every time I see someone suggesting a feature that is supported by my extension, I responded with a link to the extension.
The user base has been growing rather quickly in the last few days and is quickly approaching 2000+ users. I now have about 200 users/day signing up to use the extension.
Another growth hack that I used was this tweet:
TBH, when I first wrote this tweet, I wasn't thinking about using it to grow the extension user base. But after I saw the interest in it, I started sharing the tweet in different channels along with the link to my extension for anyone who wants to access some new features of ChatGPT now. I can't really measure how much that helped grow the user base, but considering that this tweet was viewed almost 70K times, I doubt that its effect was insignificant.
Another thing I learned was about the Chrome Extension review process. When I first launched the extension, the review process was really fast. It would usually take 2-3 hours for the extension to get published once I submit the draft. Over time, to be able to offer more complicated features like Safe Mode I had to request more permissions. I personally prefer to keep the number of permission I ask for to a minimum, but the problem with Chrome extension permissions is that they are all grouped into different categories. So if I need to use a minor feature of specific permission, I need to request access to that permission, or otherwise, I won't be able to use that feature. This slowly increased the review time for the extension, and now it usually takes somewhere from 1-2 days for it to be reviewed and published. It's not a big issue, but if I accidentally publish a new version with a bug, I'll have a stressful few hours until I fix the bug and get it published. This has forced me to do a better QA before I publish a new version of the extension.
It's been very fun working on this extension and learning more about Extension Development. I plan to continue adding more features to this in the future.
If you'd like to give it a try, the extension is called Superpower ChatGPT and you can find it here
https://chrome.google.com/webstore/detail/superpower-chatgpt/amhmeenmapldpjdedekalnfifgnpfnkc
Fun Fact: I used Midjourney to create the logo for the extension and I was very pleased with the result!
