http://www.ordnett.no/ordnett_bedrift.html
Online language and dictionary service by Kunnskapsforlaget that gives access to their major dictionaries, constantly updated (Norsk Ordbok, Fremmedord blå ordbok, Norske synonymer blå ordbok, Engelsk-norsk blå ordbok, Norsk-engelsk blå ordbok, Nynorsk ordbok). Includes Språkvett, a very useful compendium of articles on the most commonly asked questions about grammar and correct usage. Entirely in Norwegian. For Mac and Windows.
Subscription: kr398,- per year

http://www.freetranslation.com
English-Norwegian only, not Norwegian-English.

http://no.wiktionary.org is a definition dictionary, not a translating one. It gives definitions in Bokmål of Bokmål words together with the useful indefinite/definite singular/plural forms of nouns and adjectives (with comparative/superlative and derived adverb) and the tenses of verbs, any alternative spelling, Nynorsk equivalent, some synonyms and the equivalent word in a few other languages.

Format: Download
Publisher: ultralingua.com

Something a little different from the rest, Ultralingua is a dictionary for Mac computers (it's available for other platforms but not in Norwegian). For those of us who spend more time with a keyboard than a pen, this is the easiest dictionary to use. There are a few annoyances such as the collation sequence for the Norwegian entries being incorrect (å is listed with a), pronunciations being given only for the English words and, in common with most other dictionaries, the genders being given only against the Norwegian heading. One key advantage is that having looked up a word, translations can be clicked to perform the reverse look up, useful to decide which of several alternatives is the most appropriate. The dictionary has support for speech output, which is brilliant - but only as comedy!
Contributed by Joe Kelleher.

Editor: Herbert Svenkerud
Hardback
No. of pages: 1302
No. of entries: 115 000 words and 100 000 phrases and expressions
Published: 2nd edition, 1988
Publisher: Cappelen
ISBN: 9788202108236
Dimensions: 23.4 x 15.6 x 4.3 cm
Online price: kr995,- plus delivery
1st edition (1968) in Birmingham Central Library. Small font.

At the time of publication, this was the most comprehensive and authoritative Norwegian dictionary on British/American English giving nuances, phrases and idiomatic expressions with examples of usage, colloquial and regional English, slang, obsolete and modern usage. Now out-of-date but still highly rated by users.

Editor: Vibecke C.D. Haslerud
Hardback
No. of pages: 1932
No. of entries: 124 000 words and phrases. 319 000 translations
Published: 2nd edition, 2008
Publisher: Kunnskapsforlaget
ISBN: 9788257317720
Dimensions: 22 x 16 cm
Price: £103.10 (Grant & Cutler)
Online price: kr798,- plus delivery

The most comprehensive English-Norwegian dictionary available with phrases and idiomatic expressions; British/American English; up-to-date with current usage, colloquialisms, slang and technical words.