@wildwizard: Are you referring to libstdc++.so? If so it is also found in a/cxxlibs:
$ fown libstdc++.so
|| Package: ./a/cxxlibs-6.0.17-i486-1.txz
-rwxr-xr-x root/root 732224 2012-06-29 11:59 usr/lib/libstdc++.so.5.0.7
-rwxr-xr-x root/root 913916 2012-06-29 11:59 usr/lib/libstdc++.so.6.0.17
|| Package: ./d/gcc-g++-4.7.1-i486-1.txz
-rwxr-xr-x root/root 913916 2012-06-20 14:48 usr/lib/libstdc++.so.6.0.17
: Just to avoid confusion I should point out that 'fown' (find owner) is one of my own scripts, but if you use slackpkg you can do the same trick with "slackpkg file-search libstdc++.so" (IIRC) or just search in MANIFEST.bz2 (e.g. with bzless) as this is what my script and slackpkg are doing anyway.