YouTube
&autoplay=1 (append to src URL)

ie.
<embed src="http://www.youtube.com/v/qiTgKKVd9hc&autoplay=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed>

Google Video
value="http://.../googleplayer.swf?docId=-484100123711&autoPlay=true&rel=0"

(might be &autoPlay=1 instead of true)

(OR)
<embed FlashVars="autoPlay=true&initialTime=2171" (But the submit scriptFoo used to nuke this part)

theOnion
<embed src="http://www.theonion.com/content/themes/common/assets/videoplayer/flvplayer.swf" allowScriptAccess="always" type="application/x-shockwave-flash" wmode="transparent" flashvars="file=http://www.theonion.com/content...video&debugging=true&autostart=true&image= http://www.theonion.com....jpg&bufferlength=3&embedded=true&title=....0" height="355" width="400" >

*Quality work, jd. Maybe when you're done with this you can start another for finding the location of thumbnails for all supported hosts that we don't support thumbs for yet. Just a thought.

This research will help immensely. Thanks.

Awarding joedirt with one star point for this contribution to Sift Talk - declared quality by lucky760.

LIVELEAK
<object type="application/x-shockwave-flash" width="450" height="370" wmode="transparent" data="http://www.liveleak.com/player.swf?autostart=true&token=cdc_1188391804">

Revver
<embed type="application/x-shockwave-flash" src="http://flash.revver.com/player/1.0/player.swf" pluginspage="http://www.macromedia.com/go/getflashplayer" scale="noScale" salign="TL" bgcolor="#000000" flashvars="mediaId=INT&affiliateId=199&autoStart=true" allowfullscreen="true" height="392" width="480"></embed>

Where mediaId is the video ID you want to embed, and affiliateID is the affliate that gets credit for views. I recommend replacing this with a VideoSift generated account and affliate ID.

MySpace
<embed src="http://lads.myspace.com/videos/vplayer.swf" flashvars="m=16369932&a=1&type=video" type="application/x-shockwave-flash" width="430" height="346">

Son of a Bush... wish I found this sooner:
http://completetutorials.blogspot.com/2007/05/how-to-autostart-embeded-videos.html

That covers:
How-to Auto-start Break Videos
How-to Auto-start Myspace Videos
How-to Auto-start Daily Motion Videos
How-to Auto-start MSN Soapbox Videos
How-to Auto-start IFilm Videos
How-to Auto-start Metacafe Videos
How-to Auto-start Google Videos
How-to Auto-start YouTube Videos

Vimeo
<object type="application/x-shockwave-flash" width="480" height="269" data="http://vimeo.com/moogaloop.swf?clip_id=172566&server=vimeo.com &fullscreen=1&show_title=1&show_byline=1 &show_portrait=1&autoplay=1&color=00ADEF"> <param name="quality" value="best" /><param name="allowfullscreen" value="true" /><param name="movie" value="http://vimeo.com/moogaloop.swf?clip_id=172566&server=vimeo.com &fullscreen=1&show_title=1&show_byline=1 &show_portrait=1&autoplay=1&color=00ADEF" /></object>

Nice work JD!

Comment hidden because you are ignoring dag. (show it anyway)

That helps heaps - thanks JD.

The general consensus I get from googling is that Yahoo does not autostart.

VideoJug

<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://fpdownload.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=7,0,0,0" width="400" height="345" align="middle"><param name="movie" value="http://www.videojug.com/film/player?embedded=False&id=2aa1063d-cd6b-f479-4183-ff0008c9b47d" /><embed src="http://www.videojug.com/film/player?embedded=False&id=2aa1063d-cd6b-f479-4183-ff0008c9b47d" quality="high" width="400" height="345" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer"></embed></object>