You can delete a range of ZFS snapshots (a-z) in multiple ways.
The following will delete d and all earlier snapshots:
zfs destroy mypool/data@%d
To delete d and all later snapshots:
zfs destroy mypool/data@d%
To delete all dataset snapshots:
zfs destroy mypool/data@%
Make sure to let ZFS perform a dry run (-n option) first and display (-v) what
it would do to confirm that the delete operation is removing exactly what you
intended.
-- Benedict Reuschling
Don't let your zpool fill up completely by creating a dataset with
reservation.
# zfs create -o refreservation=<5% of total pool space>
You can always shrink the reserve if you need the space, but your pool will
always have space left this way.
-- Benedict Reuschling
You can disable tcsh's terminal beep if you `set nobeep'.
In order to make fetch (the FreeBSD downloading tool) ask for
username/password when it encounters a password-protected web page, you can set
the environment variable HTTP_AUTH to 'basic:*'.
Want to see how much virtual memory you're using? Just type "swapinfo" to
be shown information about the usage of your swap partitions.
You can open up a new split-screen window in (n)vi with :N or :E and then
use ^w to switch between the two.
The "zfs list" command can be filtered in multiple ways. To display just
the dataset name, use the -o parameter:
zfs list -o name mypool/usr
More columns and their order can be defined by separating them with commas:
zfs list -o mountpoint,name,avail
-- Benedict Reuschling
You can look through a file in a nice text-based interface by typing
less filename
To see the last time that you logged in, use lastlogin(8).
-- Dru
This fortune brought to you by:
$FreeBSD$
Want to strip UTF-8 BOM(Byte Order Mark) from given files?
sed -e '1s/^\xef\xbb\xbf//' < bomfile > newfile
You can use the 'fetch' command to retrieve files over ftp, http or https.
fetch 
will download the beastie image from the FreeBSD web site.
Any user that is a member of the wheel group can use "su -" to simulate
a root login. You can add a user to the wheel group by editing /etc/group.
-- Konstantinos Konstantinidis
To see the IP addresses currently set on your active interfaces, type
"ifconfig -u".
-- Dru
Time to change your password? Type "passwd" and follow the prompts.
-- Dru
If you accidentally end up inside vi, you can quit it by pressing Escape, colon
(:), q (q), bang (!) and pressing return.
Want to see how much virtual memory you're using? Just type "swapinfo" to
be shown information about the usage of your swap partitions.
You can use "pkg info" to see a list of packages you have installed.
If you want to recursively copy a directory preserving file and directory
attributes use
"cp -a source target"
-- Lars Engels
nc(1) (or netcat) is useful not only for redirecting input/output to
TCP or UDP connections, but also for proxying them with inetd(8).