wxWidgets - wxWidgets: Cross-Platform GUI Library
OptinMonster - OptinMonster helps to convert abandoning website visitors into subscribers and customers.
Electron - Build cross platform desktop apps with web technologies
Freebiebucket - Curated free tools & resources for makers 🛠
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
GetSiteControl - Website popup builder & email marketing suite for ecommerce