GssL - Full Kit
The GssL is quite possibly the most popular, audio related DIY project ever made. Originally designed with inspiration from the SSL4000 buss-compressor by Jakob Erland of Gyraf Audio, it has spun into many versions, both DIY versions and commercial products, with PCBs carrying a too-close-to-be-conincidence resemblance to Jakob's original layout.
It is a great beginners project with tons of ressources online, and you can find a million ways to mod it (if someone tells you one version is objectively "better", assume they have no idea what they are talking about!)
You can find pre-built units of various styles, looks, quality and make, but this is your chance to make and mod your own, and dive into the world of DIY with a project thats been covered and trouble shooted a million times.