Embedding a YouTube Video
Smply retrieve the "embed HTML" from your video's page on http://www.youtube.com and use it within an [html]....[/html] block.
Blah, blah, blah. [html] <iframe width="540" height="304" src="http://www.youtube.com/embed/qdxbIRkITBk" frameborder="0" allowfullscreen></iframe> [/html] More blah, blah, blah.
Blah, blah, blah.
More blah, blah, blah.
Naturally, the "blah, blah, blah" in the example is not required. It's merely there to illustrate that you can have any normal wiki content before and after the [html]...[/html] construct.
You can use a very similar method to embed Picasaweb Slideshows, or indeed, any Flash...