Granted my above idea would sort of overlap you with railscasts or asciicasts, however the one issue is that thoses casts go out of date. If your site or another site focused on idioms only, and the best practice to use those idioms, peer-reviewed with comments and maybe versioning, then it would easily become a go-to reference for developers, that would always contain the most up-to-date method of implementing said idiom.

For instance, you want to implement authentication. Right now the best thing you can do is head to railscasts, see what the latest screencast is for authenticaiton. Ah, november 2010. Might as well be ancient history! Perhaps there's a newer, easier, more-accepted authentication gem out there. Or maybe not! You just don't know. So what's next, maybe check some blogs, github, do some date-specific searches on google, etc.

If you or someone else had a site that basically said, "here's the current accepted practice for implementing authentication, and here's why", then that saves so much time!