The Subversion project website at Apache. Read release notes, documentation and find links to a wide array of other online resources.
Probably the web's best resource on Subversion, also available in print from O’Reilly Media. Recommended reading for when you have gone through the Getting started section.
A very concise and clear visual explanation of Subversion by Kalid Azad.
Excellent article by Beanstalk creator Chris Nagele on the benefits of Subversion for non-programmers. Replace the screenshots in the article with screenshots from Versions in your head to make it even better. ;)