How to add swap to existing Linux installation ?

In many embedded devices such as beagleboard, pandaboard, olimex etc Linux boots
entirely from sdcard, many times we run out of RAM and obviously forget to add
swap space during installation. The simplest way would be resizing the existing rootfs
partition and make a new partition of desired space to accommodate swap.

Add following lines to /etc/rc.local assuming you have similar nodes in /dev/ for sdcard

mkswap /dev/mmcblk0p3
swapon /dev/mmcblk0p3

Here mmcblk0p3 is the newly created partition from existing ext4/ext3 rootfs partition. This
is a dirty but quick hack to add swap to your existing image.