Archlinux have User Repository a.k.a AUR. To install from them, you can do it manually by download the PKGBUILD and makepkg to build the package and install it.UPDATE2019: yaourt no longer available. Best way to install from AUR is manually proccess..:)
But it will take time when you install AUR packages that have dependency on aur too. To make it simple, you can use Yaourt. This software will install AUR packages and also take care dependencies too.
To install Yaourt:
Install Package Query First
$wget https://aur.archlinux.org/packages/pa/package-query/package-query.tar.gz$tar -xvzf package-query.tar.gz$cd package-query$makepkg -s$sudo pacman -U *.pkg.tar.xz
Then install yaourt packages:
$wget https://aur.archlinux.org/packages/ya/yaourt/yaourt.tar.gz$tar -xvzf yaourt.tar.gz$cd yaourt$makepkg -s$sudo pacman -U *.pkg.tar.xz
$yaourt -S aurpackagename
$yaourt -U aurpackagename
Upgrade System & AUR
This will update system and also AUR packages
$yaourt -R aurpackagename
For more help
Ok that's it now you can install AUR Packages easily using yaourt command. If you want to try Pacman GUI you can install PacmanXG4.