I would suggest voogo it does exactly what you want. With help from vogoo, you can give people recommendations based on their tastes, and implement "who liked this also liked" features. I didn't try it personally but have read positive things about it. There are two versions of the software a PHP and C++ one. There are a couple of tutorials and a demo on the site, so you can check if it suits You needs.
You may also do a search for collaborative filtering, on the wikipedia page (http://en.wikipedia.org/wiki/Collaborative_filtering) for collaborative filtering You can find some interesting links to this subject.
I wanted to implement some of the collaborative search algorithms in the ecommerce site I was working on recently, but the "dead line" came to early (as always ;) and I haven't managed to implement that. Hope you have more luck :)
[edited by: lorax at 3:33 pm (utc) on Nov. 24, 2005]
[edit reason] Removed URL [/edit]
