The apps for iOS and Android are excellent and very reasonably priced.
The free online version of 'Chambers' is derived from the Chambers 21st Century Dictionary, a far cry from the Big Red Book.
The Windows-based Chambers Dictionary and Thesaurus for CD-ROM was excellent, but is now out of date (it was based on the 9th paper edition) and hard to obtain. WordWeb Pro for Windows with the Chambers add-on offers a lot of functionality, but is relatively expensive if you don't want/need the bells and whistles.