Do you restart your client often? The debug.log is just a logfile that is recording events that your client considers notable, such as restarting. You can delete that without a hickup, and the client will just start another upon the next restart. blk0001.dat is the actual blockchain while blkindex.dat is the file that your client uses to quickly find a particular transaction within all those blocks. I would still recommend that you backup your wallet.dat file before deleting anything, though.
According to my process listing (I'm running this under Debian by the way) the client has been running 179 hours, about 7.5 days when I installed it. I just left it open and running on another desktop. I just closed it out and restarted it and it cleaned up about 800MB of data from the database directory. I opened up the debug.log and it appears to be a listing of all IRC traffic that the client generates. Perhaps the next client release can clean up this file as well when the client shuts down?