http://convertcsv.com - In 2015, I averaged $650/month by using google ads.The site converts delimited (CSV,TSV,...) data to and from different formats (JSON,SQL,HTML,..). It was written in HTML/Javascript and I average about 4 hours a month supporting it.
Most comments on the site are thanking me for the site or asking for specific enhancements. So support time consists of reading comments/emails and adding enhancements and/or an occasional bug fix.