Yet another interesting LJ feature...

[kengr]

http://www.livejournal.com/friends/popwithfriends.bml

Shows usetrs and communities that your friends belong to but you don't

ETA: Drat. Only available if you've got a paid account...

Comments

Only for paid

[morgan-gw.livejournal.com]

"Finding popular users amongst your friends is a feature available to all users with paid accounts."

Oh well :-).