There was a application developed about 2 - 3 years ago that a
consumer could download that would modify their email browser.  The
aplication would modify the tool so that an ad would display in the
email when it got to it's recipient.  I want to find the name of ther
company that built this application.

Hi, mgrowsom: 
 
There were actually three competing companies doing similar things
back in the dot-com boom. One was called Epidemic Marketing, and was
based in Boulder, Colorado. You might remember the company because it
advertised during the Super Bowl back in 2000. Another was called
FaveMail (New York), and a third was called SuperSig (California).

Epidemic Marketing ended up folding in 2000 for lack of ability to get
funding, but its founder and CEO Kelly Wanser is still working in
e-mail as the CEO of ColdSpark (http://www.coldspark.com). FaveMail
eventually morphed into Expression Engines which later merged with
Bigfoot Interactive (http://www.bigfootinteractive.com). SuperSig
seems to have gone away as a company, but the firm that claims to have
built the technology for SuperSig, StickyData
(http://www.stickydata.com), has a note on its Web site saying it is
still available (link below). So, although a lot has changed since
2000 there are at least remnants of those three companies that are
accessible today.

SOURCES

A Start-Up Story, by Pamela Parker, July 28, 2000 
Internetnews.com
http://www.internetnews.com/IAR/article.php/425421 

This e-mail brought to you by ... a bad idea, by Janelle Brown February 3, 2000
Salon.com
http://dir.salon.com/tech/log/2000/02/03/epidemic/index.html

Favemail Closes $8 Million Round, atnewyork staff, April 11, 2000
AtNewYork.com 
http://www.atnewyork.com/news/article.php/338621

SuperSig Technology via StickyData's Web site
http://www.stickydata.com/tech/?subsection=ss
http://www.stickydata.com/work/?subsection=ss

Hope this information helps you! 

best,
Pamela