Travel Websites

General planning:
Wikitravel, Wikivoyage, Lonely Planet: Free online guides
UNESCO World Heritage Sites (a more detailed map here)
Sightsmap: Photographed places (useful when you zoom in closely). Similar map for sports activities here, here
Dots on Maps: Maps of places to visit.
Climate Maps: Monthly climate maps (also here, here)
Windy: Weather forecast. More here, here, here, here
Airly: Air quality map (also here, here, here, here, here)
EO Browser: Live satellite images (useful for checking snow cover). Also here
Waymarked Trails: Hiking maps. More here, here, here, here, here, here, here
Topographic maps
Soviet military topographic maps (also here, here)
Perry–Castañeda Library Map Collection: Old topographic maps. More here, here, here.
Wikiloc: Hiking trails. More here, here, here
Summitpost: Hiking and climbing. More here, here.
Happycow: Map of vegetarian food locations.
Falling Fruit: Freegan map.
Dumpstermap: Dumpster diving map
iOverlander: Practical information for overland travel
Visa requirements on Wikipedia: See your country's page for a list of countries for which you need a visa (also here)
Lonely Planet maps: Maps from Lonely Planet guidebooks (without legends).
Library Genesis: For finding travel guide ebooks (also here and here)
Nomadwiki: Useful travel tips
The Hiking Life: Good hiking blog (except the super expensive "gear" recommendations)
Travelpod: Used to be the best travel resource until it got bought and shut down by the evil Tripadvisor.
Thorn Tree forum: Another great resource, shut down in 2020.

Transport:
Rome2rio: Find bus, train, and ferry connections
Seat 61: Information about train travel
ÖPNVKarte: Maps of public transport routes
Hitchwiki: Hitchhiking guide. See their maps of hitchhiking locations (also here and here).
Flightconnections: Airline route maps (also here).
Azair: The best search engine for low-cost airlines but it seems no longer maintained. Not reliable outside Europe.
Skiplagged: Flights search engine. Good for finding "hidden-city" flights (mainly in the US)
Skyscanner: Flights search engine. Changing the country and currency sometimes gives lower prices.
Kayak: Flights search engine
Google: Flights search engine
Fly4free: Blog featuring cheap flight deals

Accommodation:
Google
Booking
Hostelworld
Open Camping Map
Trustroots: Hospitality exchange
Couchsurfing: Hospitality exchange
Airbnb: Homestays
Squat!net

Travel advisories:
https://www.gov.uk/foreign-travel-advice
https://smartraveller.gov.au/countries/
https://www.anzen.mofa.go.jp/
https://www.diplomatie.gouv.fr/fr/conseils-aux-voyageurs/
http://www.0404.go.kr/dev/main.mofa/
https://www.nederlandwereldwijd.nl/reizen/reisadviezen
https://wwwnc.cdc.gov/travel/destinations/list: Traveler's health advisory

Phone apps:
Offline maps: MyTrails, Organic Maps, Maps.me, Mapy.cz, OsmAnd, Avenza Maps (with geospatial pdfs from CalTopo).
Mobile signal coverage maps: OpenSignal.