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watch process stack, sampled at 1s intervals
This command repeatedly gets the specified process' stack using pstack (which is an insanely clever and tiny wrapper for gdb) and displays it fullscreen. Since it updates every second, you rapidly get an idea of where your program is stuck or spending time. The 'tac' is used to make the output grow down, which makes it less jumpy. If the output is too big for your screen, you can always leave the 'tac' off to see the inner calls. (Or, better yet--get a bigger screen.) Caveats: Won't work with stripped binaries and probably not well with threads, but you don't want to strip your binaries or use threads anyway.

repeat a command every one second
Short method of "while x=0; do foo ; sleep 1 ; done"

IFS - use entire lines in your for cycles
When you use a "for" construct, it cycles on every word. If you want to cycle on a line-by-line basis (and, well, you can't use xargs -n1 :D), you can set the IFS variable to .

Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.

Start an X app remotely
Launch a gui app remotely. In this example smplayer is installed on the remote machine, and movie.avi is in the remote user's home dir. Note that stdout/stderr is still local, so you'll have feedback locally, add '&>/dev/null' to suppress. This is surprisingly not well known (compared to running an X app locally via ssh -X). (NB. if your distro requires ~/.Xauthority file present, then try -fX if you have problems) Resubmitted (and trimmed, thanks sitaram) due to ridiculous voting on previous submission. Fingers crossed, it gets a better rating this time.

Insert the last argument of the previous command
for example if you did a: $ ls -la /bin/ls then $ ls !$ is equivalent to doing a $ ls /bin/ls

Show DeviceMapper names for LVM Volumes (to disambiguate iostat logs, etc)
Emits the device names which will be printed by iostat for an LVM volume; doesn't show the names for the underlying devices when snapshots are being used (the -cow and -real devices in /dev/mapper)

get a list of top 1000 sites from alexa
a little bit smarter & i only want .de domains, so .....

Repoint an existing symlink to a new location
Instead of deleting an existing symlink and then re-creating it pointing at the new location, it is possible to perform the same action with this one command. Interesting discussion on whether this is possible to do atomically here: http://answers.google.com/answers/threadview?id=526119

Bash function that saves bash functions to file from shell session
The simpler, 1-arg version is save_function(){ { date +"# %F.%T $1; declare -f "$1";}| tee -a ~/.bash_functions; }`


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