I've been using Debain UNstable with a couple of experimental packages. As you would expect, there is a pretty fast turn over of shared libraries. After a lot of 'apt-get update'ing and 'apt-get dist-upgrade'ing I've noticed that my HD usage has slowly been creeping up on me.
So, I did some digging at freshmeat and found deborphan [tribe.eu.org]. It is a small utility and really quite self explanatory once started. Basically it combs your lib files and gets rid of all the ones that no longer have any dependences.
There are two front ends for the app, orphaner and editkeep. Orphaner hacks out the dead weight, and editkeep will let you flag libraries as 'off limits'.
It worked great on my system, trimmed out a good amount of abandon libraries.
'deborphan' is even part of the standard distro, so you can get it with dselect, aptitude, or 'apt-get install deborphan'. And until you add it to the list of things to keep, it'll even sugest that you remove it ;)
One of the many wonderful things that make Debian the easiest distro to *maintain* that I've ever used.