Lost your /dev/null by accidently shoving a file there as root? Use the following to recreate:

rm /dev/null; mknod -m 666 /dev/null c 1 3

Using find on the command line as su is a great way to remove spent sessions from your /tmp directory for instance.

find -user theusersname -delete

Will remove all instances of that users files. A great quick way to clean up after a rogue application in development.

Current Directory:
sed -i s/foo/bar/g *

Recursively:
find * -type f -exec sed -i s/foo/bar/g {} \;