Google Structured Data Testing Tool
I've been spending a fair amount of time working with microdata recently and I've found this Google Structured Data Testing Tool an incredibly useful to have a tool to help me test whether or not my markup has been correctly written. This tool allows you to either analyze a URL or an HTML snippet. I've had a lot of luck copying snippets I'm working on and pasting in the tool to validate my structure. Writing the markup incorrectly doesn't exactly fail so a testing tool is essential for both learning and validation.