How much disk space takes a file or directory up?
du -s -h /your/file 
du -s -h /your/directory/

Get the 10 biggest files/folders for the current direcotry
du -sh * | sort -rh | head

Show free disk space
df -h

List only directories
ls -d */

currently mounted filesystems in nice layout
mount | column -t

Mount folder/filesystem through SSH
sshfs name@server:/path/to/folder /path/to/mount/point

Mount iso image
mount /path/to/file.iso /mnt/cdrom -o loop