The only "virus" I have ever been interested in is the hypothetical one that William Gibson. The theory in some of his work is that a virus propagated, gaining something like self-awareness, and after it crashed the net, it became the basis for AI and advanced ICE. Something like that, anyway.
I discussed with a Perlmonger friend creating a bundle with a nice script header on it that would propagate itself through out a bunch of free-host sites that allowed ftp access and/or CGI/Perl scripting/Java applications/applets. This script/bundle would behave as a neural net.
I like neural nets
The beauty part of the system would be that even if the bandwidth limit was exceeded, this emulates behavior of actual neurons, which after used for a time, lay dormant.
This is all based loosely on the theories of Chaotic Architecture by Dr. William L. Ditto, Georgia Technical Institute, School of Applied Chaos.
Of course, due to the legality issues of grabbing that much free web space from different sources, and the fact that such a project probably wouldn't garner me any money or grants, I never pursued it.
However, I believe the model is nice, as proof-of-concepts exist from P2P networks, SETI@home, and the recent parallel processing project in Canada (There's a slashdot article some time ago about the last).
While this could be construed as a "virus", I like to think of it as a self-propagating neural net/cluster. There is clustering software that does something similar, as well.