I think my favourite weekend project I've built was an image transformer plugin for Craft CMS. The popular (and only) free image plugin had been deprecated and the author was only going to continue supporting their paid version so I hacked together an Imgix-inspired plugin called Jitter[0] over the weekend. It's not very popular but I like it and use it frequently.
While it's not a very interesting project it's my favourite because of what happened after I published it. While I was at work I noticed the plugin was installed on one of our new client's websites. I went to my boss and asked him if he installed the plugin and what he thought about it. He said he did and it was better than <old plugin>. AFAIK they're still using it years later and I don't think they ever realized it's my plugin.
While it's not a very interesting project it's my favourite because of what happened after I published it. While I was at work I noticed the plugin was installed on one of our new client's websites. I went to my boss and asked him if he installed the plugin and what he thought about it. He said he did and it was better than <old plugin>. AFAIK they're still using it years later and I don't think they ever realized it's my plugin.
[0]: https://github.com/codewithkyle/craft-jitter