It's called ljwho.
It creates a tiny little icon after someone's username on LJ. If you cover the mouse over it, you get to see their "real name", and if you click on it, you can set their "real name".
The "real name" is stored locally on your own machine, and you can set it to anything you want, it's not shared with anyone.
This is extremely useful for me, since if I don't know you really well, I have trouble remembering the mapping between "person" and "lj name", and this helps a lot.
