In addition to online services such as YouTube, which allow you to share slideshows you created using other software, there are free online tools that allow you to create as well as share your slideshows.

VoiceThread

VoiceThread is a cool tool to create slideshows and comment on them. You can find it at http://voicethread.com/

With this tool you create the slideshow online by uploading the pictures and then recording the audio. It has many convenient features: for example, it automatically adjusts the duration of each picture to correspond to that of your narration; it allows you to draw on the pictures; and other users can leave comments in either written or spoken form.

The main advantage of using this tool is is ease of use. Another advantage is that it allows you to export an archival copy of each of your slideshows in the form of a movie file.  There is also an option that allows this movie to be formatted for use in iPods or iPhones. However, note that users with free accounts have to pay a small fee for each export, while thirty exports are free for users with (paid) Pro accounts.

Note that if you allow users to leave audio comments, such comments will be appended to a specific slide and played along with it. That is perfect if you are trying to facilitate asynchronus conversations on one picture at a time. However, if your slideshow depends on a linear narrative you may find this option inconvenient because it breaks the flow of your narration.

VoiceThread allows you to share PowerPoint presentations. To do that, you must save them as PDF files before uploading them. The presentations will play as slideshows and you can add commentary to them. Pro users have the option of allowing users to download and print the PDF file.  

Slideshare

You can find this tool here. To use it you must already have a presentation created with PowerPoint, OpenOffice Impress, or Keynote. You must also have an audio file (with your sound track or voiceover) in mp3 format that has been uploaded to a podcasting service or any server accessible from the web. To create this audio file you would need software such as Audacity (free download for both PC and Mac) or Garageband (Mac only; part of the iLife suite).

After registering on the Slideshare site, you can upload the presentation to Slideshare and create a "Slidecast" by linking it with your sound track. You do this by typing or pasting the URL of your audio file into the Slideshare interface. The interface also allows you to synchronize the slides with the audio in any way you like.

The drawback of this tool is that it requires you to upload the audio to a different server. If that server is down, your slideshow will not play.

Note that if you use Keynote, you will have to save your presentation as a PDF file before uploading it.