Updated 5 March 2017
For teachers
My writing reports classroom materials page
https://tefltastic.wordpress.com/worksheets/writing/reports/
Written for EFL/ ESL students
How to write a report- very short PowerPoint-style slide presentation with advice and some useful language
http://www.slideshare.net/sergeiG/how-to-write-a-report
How to write a business report- Advice and example of short but very formal business report but all information bullet pointed instead of written out in paragraphs, which is unusual and not suitable for exam tasks
https://www.thoughtco.com/how-to-write-a-business-report-1210164 – LINK FIXED
Written for native speakers
Scientific reports- long and difficult, but very thorough and good advice
https://writingcenter.unc.edu/tips-and-tools/scientific-reports/ – LINK FIXED
TEFLtastic 
Mainly for class use, my own worksheets on the topic are here:
https://tefltastic.wordpress.com/worksheets/writing/reports/