Update on 2015/12/19: Google decided to take download APK Downloader on their Chrome Webstore (after featureing it on the store for a year), so from now on you’ll have to install/update it manually. Sorry about that.
The lastest version of Official APK Downloader is here. Now supports Firefox
APK Downloader is a Google Chrome extension / Firefox addon that allows you to download Android APK files from Google Play Store (Android Market) to your PC