Good basic advice on applying for grants. From the writing center at the University of North Carolina at Chapel Hill, USA.
This section of the British Council website is aimed at linking scientists globally and providing information for international researchers on jobs and funding in the UK.
Newton's List is a tool for funders and grantseekers interested in collaborative international research and education.
From Grand Challenges Canada. Has 3 parts: Advice from Applicants, Mechanics of Writing, and Perspectives of Mentors. Includes videos and other materials.
Tutorial from the Foundation Center. Includes versions in English, Chinese, French, Portuguese, Spanish, and Russian. Focuses mainly on applying for funds from private foundations for service projects. Includes many ideas, however, that also pertain to seeking research grants from various sources.
Guidance from the US National Institutes of Health (NIH) on writing a grant proposal. Although some of the content is specific to NIH, much of it applies to writing a grant proposal for any funding source. The bulleted lists of tips seem especially helpful. (Note: If you are applying for an NIH grant, be sure to use the newest instructions for the type of grant that you are seeking. The instructions sometimes change, and so some details in "Writing Your Application" might be out of date when you apply.)