Dictionaries
Merriam-Webster
Merriam-Webster is probably the best-known dictionary of American English. It can be searched for free at www.merriam-webster.com. Search results include not just word meanings, but also examples of usage and history of the word in question.
Chambers 21st Century Dictionary
Chambers is a dictionary of British English. It is said to include more unusual words than other dictionaries of similar size, and is valued by many for the etymological information in each entry. The dictionary can be searched for free at www.chambers.co.uk. Searches yield definitions and etymology.
Style guides
The Elements of Style
William Strunk’s famous style guide was first published in 1920. The first edition is now out of copyright and can be found at www.bartleby.com. The book is now in its 4th edition; since the 2nd edition, it has been Strunk and White. EB White added to the original, but, it seems, removed little, and the 1920 edition remains useful, although occasionally dated. However, all editions of The Elements of Style are short books, focussing on use of English, and offer little guidance on many subjects covered by more recent style guides.
Australian government Style Manual
The Style Manual is available in digital format only. Although intended to aid in preparation of government publications, its two main sections (format, writing and structure; style rules and conventions) contain guidance on a wide selection of topics, many of which are relevant to scientific and technical writing.
Thesauruses
Although probably most used in the writing of fiction and general non-fiction, thesauruses can also be very useful in scientific and technical writing; how many proposal-writers have not wanted alternatives to words like ‘key’ (as in ‘key issue’), ‘approach’, ‘critical’, or ‘stakeholder’?
Fortunately both Merriam-Webster and Chambers offer free access to their thesauruses. For the curious: a quick glance at Chambers dictionary will reveal that ‘thesaurus’ comes from the Greek thesauros (treasury) and that, yes, ‘thesauri’ is also an acceptable plural.
The Moby Thesaurus is the largest English-language thesaurus. It can be searched for free here. It defines ‘synonym’ rather broadly; as a result, while Merriam-Webster and Chambers list respectively two and eight synonyms for ‘dictionary’, the Moby Thesaurus lists 38. It is perhaps best reserved for occasions when searches of the other thesauruses are unsuccessful.