Here's how to get ikiwiki in source or prepackaged form. See setup for how to use it, and be sure to add your wiki to IkiwikiUsers if you use ikiwiki.
source
Ikiwiki is developed in a git repository.
The best place to download a tarball of the latest release is from http://packages.debian.org/unstable/source/ikiwiki.
Manual installation steps and requirements are listed on the install page.
Debian / Ubuntu packages
To install with apt, if using Debian or Ubuntu:
apt-get install ikiwiki
Or download the deb from http://packages.debian.org/unstable/web/ikiwiki.
There is a backport of a recent version of ikiwiki for Debian 5.0 at http://packages.debian.org/lenny-backports/ikiwiki.
There is also an unofficial backport of ikiwiki for Ubuntu Jaunty, provided by Paweł Tęcza, at http://gpa.net.icm.edu.pl/ubuntu/.
RPM packages
Fedora versions 8 and newer have RPMs of ikiwiki available.
Ikiwiki's source includes a RPM spec file, which you can use to build your own RPM.
BSD ports
NetBSD, Mac OS X, Solaris, and many other platforms: pkgsrc has an ikiwiki package.
pkg install -U ikiwiki
FreeBSD has ikiwiki in its ports collection.
Other packages
Gentoo has an ebuild in its bug database.
The openSUSE Build Service has packages for openSUSE
A PKGBUILD for Arch Linux is in the AUR.