I have noticed over the last few months more "floaters" on sites like
yahoo.  There was one where a bunch of butterflies go across the
screen and then a flash like movied plays.  The backgroud is
transparent so you can see through the movie.   What are the
technologies for doing this.  Are they good across most browers.

Greetings!  This transparent ad technology is somewhat explained on
the Admation web site at
http://www.wincf.com/ehome/solution/#Character :

"After the development of 'WinCF', the next generation of Internet
advertising, simultaneous viewing of ads during Internet broadcasting
without added buffering time has been added. In addition, banner
advertising with moving pictures (an upgrade of existing banner
advertising technology) and translucent advertising that does not
interrupt users' work-in-progress have also developed."

I'd wager the jury is still out on the claim of "does not interrupt
users' work-in-progress..."

They also offer more on the subject at the same site:

"WinCF Semi-transparent 
Nobody expected new transparent advertising of moving pictures from
'WinCF'!
Transparent advertising is a newly developed technique that overcomes
the inherent weakness of advertising using moving pictures.
Advertisements of moving pictures that obstruct text or images on the
monitor can be made transparent so as not to bring users any
discomfort. Transparent advertising represents groundbreaking
technology for Internet advertising."

PointRoll.com (http://www.pointroll.com/Products/FreeForm.asp)
explains their FreeForms techniques:

"How It Works
When building an ad, the creative developer can select the background
of the panel to be transparent, thus allow their minds to think in
non-rectangular mode. By making the background transparent, the
designer can allocate more physical space for their panels to
strengthen their messages because the contents of the page remain
visible. The beauty of it all - the designer builds the Free•Forms
just like any other Point•Roll panel, but instead of selecting a
background color, they select the Transparent (Free•Form) option."

When I did a simple search for WinCF
(://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=WinCF), I
found only pages in other languages.  A search for "WinCF transparent
advertising" turned up only the Admation site I had already found.  In
fact, Admataion popped in in most of my searches.

I also found information at About
(http://html.miningco.com/cs/graphics/) whose site featured a link to
http://html.miningco.com/cs/gifs/, another About link page.  From
there, I found info on transparency at
http://html.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.lunaloca.com%2Ftutorials%2Fantialiasing%2F
 The techniques appears to be good across popular browsers.


SIDE NOTE:  During my searches on this topic, I located interesting
news about webbugs.  Though it is unrelated to transparent advertising
in general, it speaks of transparent bugs used for tracking an
Internet surfer's viewing habits.  I found it very informative and
thought you might enjoy reading about it, too:
http://www.crt.net.au/etopics/webbugs.htm

In part, the text I located reads "How does a Web Bug work? - These
transparent images (or Web bugs) are embedded in a Web page or an
e-mail message in the same way as any other image, inside what is
called an HTML tag. The difference with these images is that they
don’t reside on the same server as the Web page you are viewing.
Instead, they usually reside on the server of an Internet advertising
company, such as DoubleClick or MatchLogic. When viewing, a call is
made to the advertising company’s site for the GIF image to be
downloaded. This tells the advertising company that someone is
visiting the site and provides some detail about that visitor."


I hope this information proves of assistance in your understanding and
if you need a clarification before rating my answer, please ask and I
will be happy to conduct further research.


SEARCH PHRASES:

transparent Internet advertisements
transparent advertising technology
wincf transparent advertising
what is wincf
transparent banner technique
transparent banner technology

I thought I might add a little insight to this statement from
journalist-ga

"The techniques appears to be good across popular browsers."

I do agree that transparency in the form of graphics such as GIF or
PNG have evolved to a point where most browsers do display the
transparency properly, but from my own experience when the idea of a
transparent background is used in the FLASH type graphics that
ghengis-ga asked about, only Microsoft Internet Explorer seems to be
able to do this.

Here is a site that explains how to make a Flash movie transparent
http://www.kirupa.com/developer/mx/transparency.asp

This site also explains the limitations of this technique.

Limitations
While this effect may seem very useful, there are a great deal of
limitations and incompatibilities that might make you re-consider your
decision to use this effect:

Netscape browsers (even Netscape 7) will not display the effect. All
Netscape browsers simply show the animation in its opaque,
non-transparent form.
  
Most versions of Internet Explorer running on a Macintosh system will
not display the animation properly.
  
Making the animation transparent will display anything below the empty
spaces of an animation, but the Flash animation will still be stacked
higher than any surrounding object. Therefore, creating
JavaScript/DHTML menus that go below the Flash animation will not be
possible.
Granted that over 90% of all Internet surfers run a PC with Internet
Explorer 5 or higher, you should always keep in mind the 10% who might
end up being your most loyal visitors. With all that said, this effect
is great for banners and other forms of annoying, interactive
advertising.

I hope this little personal insight helps.