I don't think there is a way to do this... sometimes people make a set of their most favorited pics (favorited by other folks). I'm not sure why anyone would do this anyway, since you can see all of a person's favorites from their profile.

Can you link to an example?

If you're logged into Flickr, you should see a button on top of a picture to "Add to Favs". This will add someone else's pictures to "Your Favorites". You can see "Your Favorites" by going to the section under the "You" dropdown menu at the top of the page.

Are you sure you're not thinking of Group pools?

I'm not thinking of peoples' own pics that have been favourited (that they then make into a set, which has always struck me as a bit arrogant). I know I can see my own favourites, no prob. I'm not thinking off group pools.

I should have waited to ask this until I had an example to link to; I've seen some people make a set out of their favourites of other's pics.

You can always access the pictures some other user has favorited, through their profile. Here's terrapin's favorites, for example.

I don't think this is it, but it looks like there are a few fd toys from Big Huge Labs that do similar things.

http://bighugelabs.com/flickr/mosaic.php

Check around in the rest of them to see if any of them do what you saw:

http://bighugelabs.com/flickr/

Nope, that's not what I'm talking about. I know how to view other people's favourites, Teeps. I'm saying that I've seen people make sets out of their favourites of other peoples' pics.

Ach. Nevermind.

How about this:
http://dopiaza.org/flickr/setmgr

Oddly enough I found that by accident after following TPS's link to my own faves... I hadn't checked in a while and was curious. Found a photo of Jessamyn I liked, and clicked through to another set and she had this link in the caption.

I hope this be what you seek.