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
Done.
For Usage.
Installing packages
$yaourt -S aurpackagename
Upgrade Packages
$yaourt -U aurpackagename
Upgrade System & AUR
$yaourt -Syua
This will update system and also AUR packages
Remove packages
$yaourt -R aurpackagename
For more help
$yaourt -h
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.
Enjoy