Mount a shared folder with Ubuntu guest (and Mac OS host):

# Downloads == Name of the share in Virtualbox shared folder
# /mnt/Downloads == Directory on Ubuntu guest
# uid=1000,gid=1000 == Execute the 'id' command to get these values
sudo mount -t vboxsf Downloads /mnt/Downloads -o uid=1000,gid=1000

In a dual boot configuration with NTFS it is sometimes necessary to mount a partition as read only (for example when you have Windows 2008 quick start enabled; see also this link). The command below lets you do this:

# mount ntfs read only
sudo mount -t "ntfs" -o "uhelper=udisk2,nosuid,uid=1000,gid=1000,dmask=0077;fmask=0177",ro "/dev/sdb5" "/media//Data"