After downloading some files. Stuffit Expander crashed whilst unzipping a compressed archive. I tried again a further twice to unzip the archive and the same happened. I now have 4 files that the system refuses to trash.

Error message as follows:

Preparing to move to "trash"

 !

The item "XXX" is being used by another task right now.
(Other tasks include moving, copying, or emptying the Trash)

Try again when the other task is complete.

It sounds like stuffit expander may not have completely exited. From the terminal, you should be able to use 'ps' to find out it's process ID, then 'kill' to kill it.

ps aux | less
(note down ID)
kill ID
(if it still doesn't work)
kill -9 ID

Alternately, if it's just stuffit expander not exiting, rebooting the system should do the trick too. It's possible that stuffit expander will have a strange name under the process listing (sorry, I don't normally use OSX so I can't check) and might be hard to find.

Craig Ringer