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.
[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
[unix]$ echo filename | sed 's/file/another/'
[unix]$ sed -e 's/oldtext/newtext/' -i file1 file2 file3 ...
[unix]$ sed -e '/pattern/ d' -i file1 file2 file3 ...
[unix]$ echo FilENamE | tr A-Z a-z
[unix]$ "ls" | while read name; do mv "$name" "`echo $name | tr A-Z a-z`"; done
[unix]$ man man | col -b
[linux]$ netstat -apne --inet [freebsd]$ sockstat
[linux]$ lsof [freebsd]$ fstat
[unix]$ host 10.0.0.0 [unix]$ dig -x 10.0.0.0
[unix]$ whois -h whois.arin.net 10.0.0.0
[unix]$ dig mx some.domain.name.net
[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
Network tools
Other programs
Some links of interest