I recently learned about a tool called WAVE that allows you to install a toolbar in your browser that does accessibility checks. It also offers an online URL checking tool that lets you see your page with the accessibility features highlighted. This tool is provided by webAIM.
I have heard from people that installing the WAVE toolbar adds something Spybot considers spyware to your computer. Another tool available is Accessible Information Solutions (AIS) – Accessibility Toolbar Spybot detects nothing when the toolbar from National Information Library Service is used. There is no online URL checking tool with the AIS tool.