LibreOffice - Impress - Impress is a truly outstanding tool for creating effective multimedia presentations.
Aspose.Slides for Cloud - Use cURL commands to create, manipulate, merge, split & convert PPT PPTX presentations & slides to PDF, XPS, TIFF, ODP, OTP, HTML & other formats.
Keynote - Keynote for Mac, iOS, and iCloud lets you make dazzling presentations. Anyone can collaborate — even on a PC. And it’s compatible with Apple Pencil.
GitPitch - The Markdown Presentation Service on Git
Google Slides - Create a new presentation and edit it with others at the same time — from your computer, phone or tablet. Free with a Google account.
PPTX Viewer - PPTX Viewer is one of the lightweight applications that allows you to view, create, edit and share your presentation without facing any advertisement issue.