I regularly develop and maintain plugins at the WordPress plugin repository. These offer, like any other plugin, additional functionality for your WordPress installation. Here you find a list of my plugins which you can download completely for free.
WordPress plugin for managing feature policy headers.
WordPress plugin for receiving browser reports via a Reporting API endpoint.
All the above plugins can be used completely free forever. However, if you like any of these plugins, I would be glad if you donated some money to me. This encourages me to develop more WordPress plugins like the ones above.
Donate as little or as much as you like – these things motivate me regardless!