Not sure if blogging, collecting blogs, and interviewing people about blogging is considered a side project but ever now and again, depending on how generous the people on the other side are, I hit 500/month in donations.
Everything I do is free for everyone but for the past few years I’ve been running an entirely optional membership program that starts at $1/month.
I’m (probably naively) a big believer in kindness and I keep refusing to monetize what I do in any other way.