We build open source tools for processing spreadsheet data (
https://github.com/sheetjs/js-xlsx is our largest and most popular open source project). Our open source tools have been used by government agencies, S&P 500/Dow companies, and startups alike. We've found people have taken the library, removed header comments, and sold as-is. Some startups have taken our web demo
http://oss.sheetjs.com/, replaced the name with their own, and tried to use that to raise money. But those aren't really regrettable -- it's written off as "shameless people are shameless".
The truth is, you can never really know what would have happened if you didn't open source in the first place. Would an open source alternative emerge? Would people pay for your project or would it die in relative obscurity? That's unknowable.
If there's any regret, it's probably the amount of time spent doing support work for the open source libraries. It's not particularly interesting (usually pertaining to something in the README), time consuming (everyone seems to have their own secret project that they don't want to discuss, so you have to draw out the underlying issue slowly), and uncompensated.
The truth is, you can never really know what would have happened if you didn't open source in the first place. Would an open source alternative emerge? Would people pay for your project or would it die in relative obscurity? That's unknowable.
If there's any regret, it's probably the amount of time spent doing support work for the open source libraries. It's not particularly interesting (usually pertaining to something in the README), time consuming (everyone seems to have their own secret project that they don't want to discuss, so you have to draw out the underlying issue slowly), and uncompensated.