Aptoide-Downloader lets Linux users download *.apk files directly on their computer from aptoide.com. There's no need to install additional apps, just start the script, paste the link and you're good to go.
The *.apks are saved in your ~/Downloads directory, under the aptoide folder. (~/Downloads/aptoide)
You can also use this script on your phone. Just download it and save it somwhere convenient, then use a terminal emulator to run it. (Phone version requires Busybox and a terminal emulator, tested only on Galaxy S2.)