How can I mount a read-only filesystem as read-write?

Issue

• One of my partition has been mounted as read-only. How can I make it read-write without rebooting?
• How can I remount the root filesystem as read-write after it goes read-only?

• My filesystem went read-only, can I remount without rebooting? / filesystem suddenly became read only, unable to write to files.

  # touch file
  touch: cannot touch `file': Read-only file system
  
  # cat /proc/mounts
  rootfs / rootfs rw 0 0
  /dev/root / ext3 ro,data=ordered 0 0