I run my side project https://namesmith.io for about 5 years now. It is a domain name generator which can also check the domain availability. It is covering the server cost plus a bit extra. About $200 per month.
I didn't work on it for about 3 years but I recently decided to give it some love. I still think it has potential.
Some small earnings come from domain registration commissions themselves. Most of it comes from commissions for hosting packages.
In the past the site made more money because I got commission from domain sales/auctions. Because of API changes and what not this changed but I plan to bring this back. In my experience this is the most lucrative part of this site even if sales are very sporadic.
I didn't work on it for about 3 years but I recently decided to give it some love. I still think it has potential.