Exercise Ball

I've previously mentioned the Ab Wheel, a dirt cheap but effective way to train the abdominal muscles.

There's another cost effective piece of equipment you can buy and use at home: an exercise ball.  They are versatile, inexpensive and great for training the abs--you can lean back on it and get a great stretch.

I've included a link to one that is rated highly on Amazon.com (see below), but you may want to just go to your local retail store and look around. You may also want to check out size recommendations--the one I have linked below is 75cm, but a shorter person may do better with a 65cm (or smaller).

Note: Remember that abdominal training is only part of the equation for having visible abs (a relatively small part).  If you are wanting visible abs I'd highly recommend Mike Geary's Truth About Abs program.