Although Microsoft have got many things wrong over the years,
Microsoft Producer for PowerPoint (formerly Microsoft Producer for PowerPoint 2003) used to be a downloadable application but nowadays you can only use it online. Microsoft Producer for PowerPoint 2010 is simply designed to allow you to access your PowerPoint files on the move from wherever you are in the world as well as create basic PowerPoint presentations for free.
PowerPoint now works via Microsoft's SkyDrive and so to access your PowerPoint presentations, you'll have to upload them first which requires signing-up for a Windows Live ID. After that, using Microsoft Producer for PowerPoint 2010, you can share documents with others, collaborate on them and create basic PowerPoint presentations with ease.
There are several templates to choose from and the interface is based on the desktop version with the Office ribbon to guide you through the various functions on offer. There's also a button that takes you instantly into the desktop version if you have it installed.
Microsoft Producer for PowerPoint 2010 is an excellent albeit slightly limited tool to create PowerPoint presentations online for free.