Inspiration

Side Projects That Became Startups: 5 Inspiring Stories

calendar_todayFebruary 20, 2026personWhatToBuild Team

The most successful companies often don't start with a 50-page business plan. They start as a "scratch for an itch" – a small tool built on evenings and weekends.

Here are 5 world-changing startups that began as simple side projects.

1. Slack (Internal Tool)

Before it was the world's communication hub, Slack was an internal tool built by a gaming company called Tiny Speck. They were building a game called Glitch, which eventually failed. However, the chat tool they built for their own developers became Slack.

Lesson: The tool you build to help you build your product might actually be the product.

2. Unsplash (Leftover Photo Assets)

Unsplash started as a side project of the crew agency to use up leftover photos from a professional shoot. They put them on a simple Tumblr blog and offered them for free. It was so popular it crashed their site and eventually became its own massive platform.

Lesson: Generosity can be a marketing strategy.

3. Product Hunt (Email List)

Ryan Hoover didn't build a complex site on day one. Product Hunt started as a Linky (email list) where he and his friends shared cool products they found. The engagement was so high that he eventually turned it into the site we know today.

Lesson: Validate with the simplest possible version (MVP).

4. GitHub (The Weekend Project)

Chris Wanstrath and PJ Hyett were working at CNET and wanted a better way to share code and collaborate with Git. They spent weekends building a "social network for code." They launched it in 2008, and it eventually sold to Microsoft for $7.5 billion.

Lesson: Build the tools you wish existed for your own craft.

5. Craigslist (Community Email)

Craig Newmark started Craigslist in 1995 as an email distribution list to friends, highlighting local events in San Francisco. It grew through word of mouth and eventually expanded into the minimalist behemoth it is today.

Lesson: Keep things simple and focus on the utility.


What's your side project? Every entry on the WhatToBuild Feed has the potential to be one of these stories. The key is to start building.

Feel free to post a problem if you see a friction point in your daily life – you might just be starting the next big thing.