The Purdue Online Writing Lab (OWL)
For writing business documents, placing your commas, following a citation style, and any other queries you want to enter into the handy search box.

The Elements of Style by William Strunk, Jr.
This influential guide plainly illustrates some good practices for writing clearly and using rules consistently. Just reading through the contents page is already an education!

about.com’s ‘Glossary of Usage: Index of Commonly Confused Words’
The difference between ‘every day’ and ‘everyday’, ‘principal’ and ‘principle’, and many more.

Oxford Dictionaries
Set the dictionary to UK English or other options with the blue drop-down menu in the search box.

Merriam-Webster
For US English.

LEO
German <> English as well as German <> various other languages. Available as a mobile app. Creating an account makes it possible to save looked-up words.

Linguee
Translated text extracts in two languages (e.g. English and German) can be compared to get a sense for how a word or phrase is used. Do however keep in mind that many of the translations have not been vetted.

Coursera
High-Impact Business Writing by University of California, Irvine
This course is a terrific primer in the basics of business writing. At the moment there are a few minor glitches in the videos, but it doesn’t make a big difference. Although focused on business writing, many of the basic principles are also useful for academic writing. See some of my takeaways from this course on the blog.

Toastmasters International
If you want to improve your presentation and general oral communication skills, joining a Toastmasters club is one of the simplest and most fun ways to do so. Regular meetings include opportunities for prepared, unprepared, and semi-prepared speaking. You build on the skills you already have with feedback from your peers. Before becoming a paying member, you can visit any club in the world as many times as you like for free to try it out. I highly recommend Toastmasters from my personal experience.

lang-8.com
On this platform, you can practise writing in English – or any language – and be corrected by native speakers. It’s a useful free way for regular training. Do remember, however, that everyone who corrects your work is not a language professional. Also keep in mind that interesting content is likely to get more readers. All in all the platform is a fun place for exchanging knowledge, receiving what you want to know about a foreign language and giving what you know of your native language.

ShaunaKelly.com
If you’re not using heading styles in Microsoft Word or similar software, you should learn how to do so today. Check out Shauna Kelly’s easy-to-use guide to heading styles. For longer documents like theses or reports, also check out how to set up numbering with heading styles.