These are some simple things, don't expect anything fancy. Also, realize that most of these things can be done multiple ways and with multiple languages.

How to run a command to a set of files (The while read command can handle names with spaces but you need to quote the variable after):
[unix]$ for i in `ls *mp3`; do cp $i $i.bak; done
[unix]$ ls *mp3 | while read i; do cp "$i" "$i with more spaces"; done
How to rename part of a file:
[unix]$ echo filename | sed 's/file/another/'
How to replace text in multiple files:
[unix]$ sed -e 's/oldtext/newtext/' -i file1 file2 file3 ...
How to delete lines matching a pattern in some files:
[unix]$ sed -e '/pattern/ d' -i file1 file2 file3 ...
How to change all letters to lowercase:
[unix]$ echo FilENamE | tr A-Z a-z
A good way to change all filenames in a directory to lowercase:
This handles a colored ls (hence the quotes around ls, but you can use any file list output, i.e. find) and names with spaces (hence the while read and not the for).
[unix]$ "ls" | while read name; do mv "$name" "`echo $name | tr A-Z a-z`"; done
How to convert man pages to text files:
[unix]$ man man | col -b
How to list all the open ports and programs:
[linux]$ netstat -apne --inet
[freebsd]$ sockstat
How to list all the open files and programs:
[linux]$ lsof
[freebsd]$ fstat
How to find information about an ip (i.e. 10.0.0.0):
[unix]$ host 10.0.0.0
[unix]$ dig -x 10.0.0.0
How to find information about an ip owner (i.e. 10.0.0.0):
[unix]$ whois -h whois.arin.net 10.0.0.0
How to find out the mailserver for an ip:
[unix]$ dig mx some.domain.name.net
How to generate a filename with yesterdays date (i.e. for logfiles/backups):
[linux]$ touch logfile-`date -dyesterday '+%Y-%m-%d'`
[freebsd]$ touch logfile-`date -v-1d '+%Y-%m-%d'`


Commands/options you should know about, read the man pages

tail - display the end of a file
head - display the beggining of a file
xargs - convert stdin to command line arguments
nmap - network scanner
watch (linux) - run a command periodically and watch the output
watch (freebsd) - monitor a tty
lsof - lists the open files
strace (linux) - trace the system calls of a program
truss (freebsd) - trace the system calls of a program
calc - command line calculator program
bc - another command line calculator program
dc - command line RPN calculator program
du - how much disk space is being used by something
df - disk free, how much free space there is
touch - creates an epty file or updates the timestamp on a file
diff - show the difference between 2 files
strings - show the strings in a file (useful for binaries)
file - tells you what type a file is
finger - find out information about users
w / who - who is currently on the machine and what are they doing
id - tells you who you are
uname - tells you basic information about the system.
whois - information about hosts
crontab - execute commands at specified times
last - last users to log into the system
mail - very basic mailer, useful for redirection
nice/renice - set the niceness ("priority") level of a program
nohup - leave a program running after logging off
screen - virtual terminal for leaving programs running
psnup - print multiple pages in one page
time - time the execution of a command
wc - word count, tells you the size in bytes, words and lines
which / whereis - locates commands
locate - locates files
whatis - gives brief summary of a command
wget / fetch - get files off the net
talk - chat with other users on the system
write / wall - send messages to users of the system
zcat - cat gzipped files
ln - link files
sleep - wait a specified amount of time
md5sum (linux) - calculate the md5 signature of a file
md5 (freebsd) - calculate the md5 signature of a file
enscript - convert text files to ps
awk, sed - useful scripting programs
cat, echo, more/less, find, grep - you need to know these, it's not an option.
Network tools

netstat - network information
ping - check if hosts are up
tcpdump - look at the traffic going through the network
nmap - scan hosts
finger - see info about remote users
whois - get info about hosts
ifconfig - configure interfaces
ifup/ifdown - script/program to enable interfaces
Other programs

ispell/aspell - spell checkers
dict - dictionary tool
Ref: http://sluglug.ucsc.edu/~isolis/knowledge/unix-tips/