Free
Norsk ordbok (Nynorsk and dialects. Brilliant, but lack of funding means it’s missing A to H.)
Lexin (Bokmål/Nynorsk to Arabic, Dari, Kurdish, Persian, Polish, Russian, Somali, Tagalog, Tamil, Thai, Tigrinya, Turkish, Urdu and Vietnamese. Also Bokmål to Nynorsk.)
Wiktionary Bokmål / Nynorsk / English
Akademisk ordliste (750 Norwegian words you should know as a student.)
Islex (Icelandic <–> Bokmål, Nynorsk, Swedish, Danish, Faroese.)
Tvärslå (Lots of dictionaries combined into one search: Bokmål, Nynorsk, Danish, Swedish, Icelandic, English.)
Den danske ordbog (Danish)
Svenska Akademiens ordlista (Swedish — Svensk ordbok app (65k words), Svenska Akademiens ordlista app (125k words), Svenska Akademiens ordlista on-line)
Yourdictionary.com (A collection of links to dictionaries in 300 languages)
Cercurius (More than 2000 links to dictionaries, grammars etc. Swedish.)
Not free
Ordnett (Norwegian <–> English, Swedish, German, French, Spanish, Italian, Russian, Portuguese, Chinese.)