At zoho.com you'll find an entire suite of web-based productivity tools that in some measure replicate and in other ways even supercede the functionality of the familiar Office products from Microsoft. Zoho Show is the the tool in the suite that emulates PowerPoint. Using Zoho Show you can convert a .ppt presentation to a web-friendly format that you can easily embed into most web pages. Accounts are free and very easy to set up.
Here is how my presentation looks uploaded to Zoho and embedded in this page:
The conversion process involves the usual flattening, but is otherwise fairly robust. The end-result can be viewed independently or embedded into a web page as below. It's incredibly convenient for browsers who can either view the show without leaving your page or toggle to 'full-screen' mode by clicking the rectangle in the lower right. Embedded shows are not only convenient and attractive, they are also relatively immune to tampering. If you include copyright information on your slides, it will not be easy for someone else to delete.
A note on BlackBoard: The embedding technique is not advisable for including Soho shows in a course Blackboard. Instead you can create an external link in the Blackboard to the public url for your show and then select the "Special Action" to "Display media file within the page." Without leaving blackboard, students will be able to view your show and even leave comments. Alternatively, you could use a service whose embedded shows rely on <object> or <embed> tags rather than on an <iframe>.
The disadvantage of linking or embedding—there's always a disadvantage—is that you will have to make your show open to the world. For an interesting, if slightly involved alternative, view the screen on the Zoho export to html.
When you upload a PowerPoint file to Zoho, you'll have the option to make your converted file editable or read-only (the recommended option).
Personally, I find the option to create the presentation directly in Zoho rather attractive. The editing interface is limited compared to a desktop app, but you will automatically be prevented from creating anything that will not convert. Additionally, you will be able to edit your presentation even after you have embedded it. Any changes you make subsequent to embedding will automatically be reflected in future viewings. Below is a show that I created within Zoho.
If the authoring functionality of Zoho is less important to you than opportunities to connect with others, then you may be more interested in a service like slideshare.net.
If privacy is an issue because of copyrighted material, you can use Zoho's "Export to HTML" option.