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
Sleeknote - Set up lead capture forms on your website in minutes. No coding required. Sync your new email leads with your favorite email service provider. Try for free.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Poptin - Create engaging and high converting website pop ups and contact forms in minutes. With Poptin, you get access to a feature-packed conversion rate optimization toolkit that converts more website visitors into email subscribers, leads, and customers.