Of the side projects over the years and the ones that I've stuck with are the ones I do because it's fun and rewarding. Not to say money makes a project bad, but make sure you like what your building before being motivated by finance.
About 3.5 years ago I started https://fuse.blog a simple list of interesting links that I posted to daily. Today I launched a brand new site and opened the archive for the first time.
Although I haven't made money directly, the project was a vehicle to practicing design and learning to code, which has attracted clients and made value in other ways.
What do you think about non-monetary side-projects?