Recommenders - What?

I just finished reading Rita Kop's blog post called The eXTended Web and PLEs. One of things she talks about are Recommenders. I think that the idea of having a tool / resource that would recommend things I was interested in would be great. But what does a recommender look like? Do they already exist? If so, what do they look like?

Thinking of my PLE, I tried to identify what, if any recommenders did I have in there?

•  Friends and Colleagues - they know my interests and often share links, movie clips etc 
• The Oldaily.  To me, this is a recommender.  I am interested in many of the same things that Stephen is, so when he thinks a link is good enough to be placed in the Oldaily, I take notice.
• My RSS feeds, again, to me, these are recommenders.  I don't have a lot of them, but I am very interested in what ever they turn up.
• My Twitter Searches - I have a coupe of twitter searches set-up in Netvibes which feed me twitters based on hash tags (#plenk2010 is one of them).  Again, these are to me recommenders.

What are some more recommenders out there?