Pádraig Brady has some useful, if somewhat basic hints, at http://www.pixelbeat.org/cmdline.html. He has updated them to include more powerful commands at http://www.pixelbeat.org/docs/linux_commands.html.
Here are a few of my favourites (I have taken the liberty of slightly altering some of the code and/or descriptions):
From the original:
Search recursively for “expr” in all *.c and *.h files:
find -name '*.[ch]' | xargs grep -E 'expr'
Concatenate lines with training backslash:
sed ':a; /\\$/N; s/\\\n//; ta'
Delete line 42 from .known_hosts:
sed -i 42d ~/.ssh/known_hosts
From the new post:
Echo the path one item per line (assumes GNU tr):
echo $PATH | tr : '\n'
Top for Network:
Top for Input/Output (I/O):
Get SSL website Certificate:
openssl s_client -connect http://www.google.com:443 < /dev/null
List processes with Port 80 open:
lsof -i tcp:80
Edit a remote file directly in vim:
Add 20ms latency to loopback device (for testing):
tc qdisc add dev lo root handle 1:0 netem delay 20msec
Remove the latency:
tc qdisc del dev lo root