In Two and a Half Men, Walden is an internet billionaire because Microsoft had bought his site and used it for their iPod killer, Zune, earning him a billion dollar bank account! What was this site he created?
As I wrote in the comments, the site was called BlunGoGo. It's explained in the season 9 opener:
Alan: So, just out of curiosity, how does someone get to be worth so much money?
Walden: Well, it's pretty simple really. You ever hear of BlunGogo.com?
Walden: You never will, because Microsoft bought it from me for $1.3 Billion. Then they bundled it with their "iPod killer," the Zune.
...and to the best of my knowledge, that's unfortunately all we know about it.