Xtreme Download Manager. Cross platform clone of IDM and insanely valuable if you are a regular downloader. (Though I would argue IDM is better and more polished if you mainly use Windows.)
JDownloader. Cross platform. Do you have dozens of annoying file sharing website links (1fichier, MediaFire, MEGA, Gofile, Sendspace, Turbobit, UploadRar) with even more annoying time limits between downloads? Do you find file sharing websites with ads and popups annoying? Throw all your links into this tool. Start. Go sleep. Do your work.
MediaInfo. Cross platform, and allows to view every possible detail about video and audio files. This is for nerds and media encoder nutjobs like me.
Calibre. Cross platform. THE BEST E-BOOK READER AND ORGANISER ON EARTH.
Bonus for Linux: Thunar is the best file manager, if you do not want a complicated dual pane TC style tool. Nautilus is a close second, because they refuse to have Backspace key as directory up shortcut.