A Web Mural is a webpage that is sold by the pixel. The sets of pixels show different people's images, with each square being a link to the website/webpage of their choice.
The result is an interesting, colorful, informative piece of functional contemporary art.
The web mural that started them all was created in 2005 by a 21 year-old British man named Alex Tew, who need money for college. It was wildly successful -- he made USD$1,037,000 in just under five months!!! The idea spurred a new method of internet advertising, too.
Alex's page is called MillionDollarHomepage.com
(check out his FAQ's, too, interesting story)
Here is the wiki article
Here is what the original mural looked like:

All of the images are links. Web murals have been used as fundraisers as well as social/political statements of the time.
There are many reasons to buy a square of pixels on a web mural:
* advertise a business/organization with a permalink
* generate interest in and traffic for your business/organization
* show your support NYCabs' vision for better transportation
* display a public statement of some kind
* display your own artistic image and link to their own site/myspace page
* display a statement of love, friendship, or memorial
* participate in something larger than yourself
* a fun way to donate to a worthy cause
* your message and link will be forever encased in a contemporary piece of
art that is publicly displayed
* make a statement that the public is serious about public transportation