I found some really cheap macro extension tubes for Canon on ebay.

http://cgi.ebay.com/Macro-Extension-Tube-for-Canon-Camera-Lens-SLR-DSLR_W0QQitemZ260339310215QQihZ016QQcategoryZ64345QQcmdZViewItem?refid=store

The one I got was $21 with free shipping.

Extension tubes are a great way to take great macro and extreme close up photos using the lenses you have already. They are also cheap because they have no glass in them, they are just hallow tubes.

The only drawback on this one is it's built pretty cheap and it does not communicate with your camera so you have to figure out everything manually.

I have some more info on my blog.