This folder will be about 12GB currently, and is always growing in size as the blockchain grows from use.
There may be some files that can be removed if either:
1. You were previously using Bitcoin before
0.8.02. You used the bootstrap torrent to initialize the client.
Files that can be removed if they exist in the root directory (don't touch blocks, chainstate, or database subdirectories):
You can remove blk0001.dat, blk0002.dat, blk0003.dat and blkindex.dat from the root data directory after a reindex from installation of 0.8.0+ is complete and you are caught up with the blockchain (and you don't plan on going back to an older version).
You can remove any bootstrap.xxx file from this directory after it has been used.