HTML5 Please – Which Features Can You Use?
HTML5 Please is a great resource for determining browser support for many HTML5 and CSS3 Features. Each feature offers a recommendation to either use, not use, use with caution, or use with a fallback. It offers great information like the browser share for each feature, as well as the fallbacks you can use to simulate the feature in incompatible browsers.
My favorite feature is the search, which allows you to filter features by compatibility, for instance what is safe to use in IE8+.