[poooogles]

The Python REPL and the JSON lib. Probably not the most efficient if you're starting out but if you're familiar with Python it can be quite effective.

[ezstar]

This. I use the Ruby REPL, but same principle.

You have to be a little careful about not accidentally printing out the whole thing to stdout, but after its loaded into memory you can check keys at a given level, drill into the substructure, etc.