Finding missing packages - three methods

Sometimes, when compiling applications, rather than installing them from packages, one comes up against a problem like this one recently posed in #kubuntu (IRC channel on Freenode):

Hi, I have an error then I'm compiling an app. It doesn't find /usr/lib/libGL.so. How can I install it?

Here is one answer:

[01:45] you want to install libgl1-mesa-dev
[01:46] you also want to install apt-file, 'apt-file find /usr/lib/libGL.so' will tell you that you need libgl1-mesa-dev for it

Here is the second way:

[01:54] you know, packages.ubuntu.com has the same function
[01:54] sadly doesn't work with PPAs though since the LP folks didn't implement the Contents.gz it needs yet

And the easiest way if you are in #ubuntu or #kubuntu (or any channel with ubottu). Notice there are two ways to ask:

[01:54] as does ubottu :)
[01:55] !find libGL.so
[01:55] File libGL.so found in fglrx, ia32-libs, libgl1-mesa-dev, libgl1-mesa-glx, libgl1-mesa-glx-dbg, libgl1-mesa-swx11, libgl1-mesa-swx11-dbg, libgl1-mesa-swx11-dev, lsb-build-base3, nvidia-173 (and 2 others) http://packages.ubuntu.com/search?searchon=contents&keywords=libGL.so&mode=&suite=maverick&arch=any

[01:56] !find usr/lib/libGL.so
[01:56] File usr/lib/libGL.so found in libgl1-mesa-dev

Slick! Thanks to yofel and tsimpson for their help.