On a Debian based Linux distribution like Crunchbang, it's quite easy to determine which package a specific file on the system belongs to. Issuing dpkg-query -S /usr/lib32/gconv/UNICODE.so on my desktop system tells me, that the file belongs to the libc6-i386 package - the 32 bit shared libraries for AMD64.
Sometimes, however, it comes handy to know what package a file not present on the filesystem belongs to. One prominent example is the tunctl program, that allows the creation of TUN/TAP interfaces on Linux. A search for tunctl via aptitude search tunctl doesn't yield any results. That's where apt-file comes into play. After installing it via aptitude install apt-file and updating the index with apt-file update, we can start a more advanced search. Using apt-file find tunctl, we get the info that the file /usr/sbin/tunctl is included in the uml-utilities package. To allow more sophisticated searches, apt-file offers various advanced options, e.g., for case insensitive searches based on regular expressions.