Recently I needed to figure out what was taking up so much space on my Android phone to the point where I only had a couple hundred megabytes of free space.
DiskUsage is a free app that will scan your storage and then display a map of all your directories, making it easy to find what is eating up all that space.
It worked well, and I was able to find and prune the offending subdirectory quickly. Just a nice, task-specific utility to have.