tcpdump

for ftp traffic
tcpdump -A -i eth0 tcp port 21

comes in handy when you want to see what passwords are used.

filenames beginning with a - (dash)

If you want to (for example) cd to a file starting with a dash (some mac users tend to use weird filenames like "-folder" ). do

$cd ./-folder

removing files matching a certain pattern

from the xargs man:

delete all .svn folder

$find /somedir -name .svn -print0 | xargs -0 /bin/rm -rf

xargs comes in handy when handling great amounts of files.