cmake at least version 2.4.3.
For many popular distributions cmake is incorporated by
default or at least apt-get (or whatever your distro package installer
might be) is cmake aware.
Still, in case you need to install an up-to-date version cmake's
official site distributes many binary versions (alas packaged as tarballs)
which are made available at
http://www.cmake.org/HTML/Download.html.
Optionally, you can download the sources and recompile them: this simple
process (./bootstrap; make; make install) is described at
http://www.cmake.org/HTML/Install.html.