WebAIM is an invaluable site with accessibility tools, research, and other resources. Here are some highlights:

• WAVE, their Web Accessibility Evaluation Tool, allows you to quickly and easily analyze your site for accessibility issues. There are also WAVE browser extensions for Firefox and Chrome.
• The Color Contrast Checker allows you to plug in foreground and background colors and get back a contrast ratio as well as pass/fail determinations for various text legibility guidelines.
• Web Accessibility for Designers is a great infographic for anyone that wants to improve the accessibility of what they create.
• The WCAG 2.0 Checklist and the Section 508 Checklist are great resources for anyone that creates things on the web.