Zoho Show is an online tool to create, edit, publish, and show presentations. It's web-based. So, you don't need to download or install anything. All you need is a browser (Firefox 1.0 + or IE 6.0+ or Mozilla) and internet connection.
Announcing Vyew 2.0: Free Web Conferencing and Always-On Collaboration * 100% Browser-based - No downloads or installs * Shared viewing of: DOCs, PPTs, XLSs, JPGs, PDFs and more * Real-time Desktop Sharing and Screen Capturing * Tools to Whiteboard, Draw