Academic

Purdue University Online Writing Lab (OWL)
The most famous Online Writing Lab.

Mantex - Free Downloads
Follow the links from the Mantex site for tips on essay planning & presentation, how to summarize and use quotations, science report writing...

Writing Your Research Proposal
A workbook for first time and inexperienced researchers in the social sciences and humanities. It includes useful advice about how to research, plan, structure and write a research proposal. 

Spelling

Tiscali reference
A good summary of spelling rules from the Hutchinson Encyclopaedia published on Tiscali's reference site.

General

BBC World Service support material
For help with writing business reports, try the 'Professional Skills' study guide in the support material section.