2.Add-ons: Give the base product for free and sell add-on modules. OrangeHRM is the best example. OrangeHRM is free to download and you can customize it yourself or buy the add ons as per your business needs.
3.Code contribution/Testing : Open source projects owners look for programmers or testers who can support their projects. All you need to do is pick the right product and work for them. You may get paid.
4.Donation: Most open source products have a link for donation. One can make really good money on this. If your product is fruitful for users they don’t mind to donate to keep the project going on.