Not sure of the policies on necroposting here but what the hell. The single most useful app is your web browser (I prefer Opera but the choice is yours) because its the app which allows you to obtain every other app. After that I'd say your torrent client (mine is utorrent) and jdownloader, for similar reasons (but you need to browse to find the torrents and ddl links), WinRAR and Daemon Tools and ImgBurn for dealing with files you've downloaded. And after that it fans out depending on what it is you're into.
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