There is now a new version of phpLD that allows you to run a site that allows people to bid for position on their links. While this is still new product, it seems to be working flawlessly for people that have tried it so far, and it even seems people are making money from it already.

More Info: Here

This is a free download, but we ask that you retain a link back to phpLD, or pay $50 for link removal.

Here are some example sites that are running it:

LinkBids Web Directory
Unique Bid Directory
Signitas Bid Directory
Tag Links
Ranking King