Bashcomp (app-shells/bashcomp) is a very useful tool that takes advantage of Bash's auto-completion and allows it to be applied to other programs.

Installation is straight-forward under Gentoo, although its useful to set a USE=bashcomp for the app-admin/eselect package so…

echo 'app-admin/eselect bash-completion' >> /etc/portage/package.use
emerge -av eselect bashcomp

Once installed its relatively straight-forward to enable Bash-Completion once installed. To get a list of programs that support bash completion type the following as root…

eselect bashcomp list
Available completions:
  [1]   ant
  [2]   apache2ctl
  [3]   apt
  [4]   apt-build
  [5]   aptitude
  [6]   aspell
  [7]   autorpm
  [8]   base
  [9]   bash-builtins
  [10]  bind-utils
  [11]  bitkeeper
  [12]  bittorrent
  [13]  bluez-utils
  [14]  brctl
  [15]  bzip2
  ...

To enable globally (i.e. for all users) bash completion for a particular program then type as root…

eselect bashcomp enable --global [program]

Thats it, you've enabled bash completion for that program.