I've started several open source projects.
I have one that I started in 2002 and have continued to use myself over the years. Its an xml parser that works the way I think it should work. I wrote it and made it open source because I found myself wanting to use xml in projects at various employers and it was always a hassle finding an xml parser and convincing management to allow me to use it (because open source) and then it never had exactly the right features, etc. Xerces was too big, etc.
So, finally I just wrote my own. Since I own the copyright, I can use it and grant the employer a non-exclusive close source license if they want.