I started two projects last year and now make ~$100/month in profit. Costs are almost nothing.

https://autoplaylists.simon.codes: iTunes-style smart playlists for Google Music.

https://gchat.simon.codes: autoreplies for Google Chat/Hangouts. Particularly useful for people with multiple accounts that forward email.

https://simon.codes/2017/01/09/side-project-income-2016-0-to... has more details on why I chose these and how I got started.