If it's a pdf-form, you can use Acrobat reader.
If it's an spreadsheet, you can use Excel or Open Office.
If it's an html-form, you can (in principle) use a browser.
If it's a Word-document, you can use Word or Open Office.
In general I'm inclined to say: follow the instructions of the organization you want to apply to.
You might be interested in a product (costing $245, substantial discounts for schools and non-profit organisations ) called Form Pilot Office 1.94 that's designed for filling out electronic forms of any type. See http://www.colorpilot.com/
Hope this helps.
Is it possible to use one of the office applications to fill out job applications forms once you have downloaded them to your computer?
If so which one or does anyone know of some other software that can be used?