If you find that you are running out of disk space and some of your applications crashed recently, you might want to look in your home folder. When applications crashes, Ubuntu creates here memory dumps to help the programmer determine the cause of the crash. But if you are no programmer, you can safely delete these files.
When you have problems with locked files / devices, use lsof (List Open Files). And remember: in Linux everything is a file. Example: you want to play a media file but your player complains that it can't open the playback device (because it crashed earlier and probably left it open). Go in the console and type
lsof | less and go through the list looking for
/dev/snd. You should see the program that's holding it open so that you can do a kill on it. If you don't have lsof installed, just do a
sudo apt-get install lsof.
Update: please read the followup on this advice.