SmartForms is a free form as a service product that allows you to submit HTML forms without a backend, you can do this simply by creating a form endpoint and pointing it in your HTML form. SmartForms also allows you to receive notifications when someone submits on one of your forms in multiple channels: E-mails, Slack, Telegram and Push Notifications. List of features:

Create as many form endpoints as you want Handle file upload, save files on the cloud so you can download later Get notified on every submission in E-mail, Telegram, Slack and Push Notification. Super easy to set-up. Spam protection so your inbox is clean and only with what matters Customizable auto-response page and e-mail [optional] AJAX Friendly Team collaboration RESTful API for accessing your submission data Submissions panel where you can read, delete, manage spams, etc

The product has a generous free-tier, the best on the market and the paid plans have fair pricings.