I find there are often a lack of examples how to do some specific thing using a particular API or framework. A well indexed and user-taggable code store could solve this, especially if it is made very easy to contribute code. This is good for two categories of code:
'deep magic' and 'straightforward but non-obvious'

and also for analyzing the usefulness of varying ways to model a given problem.

For example, metaclasses in Python (straightforward but non-obvious) really couldn't possibly have too many examples available.

What I really want myself is a unified code search engine.. google sort of does this, but snippets and tagging are essential IMO.