When we,
./configure
make
make install
where are the programs installed?
As Dirk notes, the default prefix is "/usr/local"; however, you can change it. For example:
./configure --prefix=/opt/local
make
sudo make install
Note, though, that you should not install software in this way. You should use Ubuntu's package management system called apt-get to install software. You should only use configure+make+make install if there is no apt-get repository containing a package for it. The reason you should use apt-get is that it automatically manages dependencies and versioning of software and will ensure that your software is automatically updated. Installing things manually is a good way to show you do not care by introducing dependency conflicts or a good way to have outdated, possibly vulnerable software on your system. So, before you install something that way, you should use apt-cache search to find out of a package already exists, and then you can use sudo apt-get install to install it. Example:
apt-cache search boost # This will show all sorts of packages related to Boost
sudo apt-get install libboost-dev # Ok, this is the one on the list I want
Default prefix (or destdir) is /usr/local unless that has been overridden in the autoconf logic.
You can often override this in the make install step too.