QueryPosts.com is a great site for documentation and source details on WordPress functions.
WordPress.org’s Codex is an essential site for referencing the inner workings of WordPress and does have a long list of documentation covering most of the WordPress functions, but does not cover all the functions available in WordPress and doesn’t show the functions’ code either. Enter QueryPosts.com, which covers a great deal more of the functions available in WordPress and shows the functions’ source. Bookmarking QueryPosts.com can really help when you are struggling to find that exact function in WordPress.
Also, WordPress at codexamples.com is very helpful too, with the same concept of showing the functions source.
If you have another site that is a great reference for WordPress development, please submit a comment below.