Arch User Repository. If you’re using Arch, you get the basic stuff from the official repositories. But for most programs there’s the AUR. They’re often less polished, some of it may be proprietary. There are package managers dedicated for it, that also know to handle the official repositories.
I am quite comfortable finding my way around ArchLinux, and recently decided to give Gentoo a try. I didn’t expect it to be that much harder but all the cflags, emerge, conflicts and updates feels like black magic. I guess that if you know your way around Gentoo, reverse-engineering a deb file is not a real challenge. However I’m assuming that most Linux users would hope for a less involved solution.