Libreoffice – the best, most customisable and powerful office software available
Onlyoffice – alternative for less-advanced users who are used to the UI of contemporary MSO
Zotero – great bibliography manager useful when writing scientific papers: lets you collect books, journal articles and all other types of sources, automatically finds full text PDFs online, fills in metadata and then inserts dynamic citations in thousands of different, customisable styles. Also generates bibliographies. Works with LO, MSO and GDocs
TeXStudio – my LATeX editor of choice; integral (ha!) when formatting maths-heavy documents
Android:
Cloudstream — free streaming app, works with SFlix, Sodastream, PH and other legally dubious streaming providers. Takes some trickery to set up though.
Osmand — OpenStreetMap client with offline (optional online) navigation and plenty of plugins; loads of customisation
Material Files — nicest file manager, especially for rooted devices
Showly — freemium open-source TV and film tracker. Syncs with Trakt.tv
Simple Gallery — out of all Simple Apps by this developer, this is the only one which is in fact superior to its alternatives. Highly customisable, powerful, lightweight gallery app
PC:
Android: