plv8.

    https://code.google.com/p/plv8js/

Besides the obvious (such as projecting a sub-datum from a json datum), you can also do data transformations (whatever javascript can give you) -- as a contrived example, the sine of a number or the sum of two numbers -- and, you can try projecting arrays or hstores from a json and then using GIN/GiST to do a more expensive but more powerful index over them.

A quick and dirty example from some time ago: https://gist.github.com/1152997

Some more exotic built-in indexing will probably show up in the coming years, but a lot of things can be accomplished relatively soon with this extension.

You kind of can using partial function indexes already but it's not very flexible.