I started this as an email to my dad but then thought, why not share it here? I have some drawers that are too deep. I saw pictures of units with lots of shallow drawers before I built this drawer unit but, just couldn't get my head around it. It turns out that lots of shallow drawers are actually more useful than fewer deep drawers in some cases.
At any rate I wanted more storage in some of my too deep drawers and dad had expressed the same desire in his shop. Here is a super simple solution that is laying around in your scrap bin.
I made two strips about 1-3/8" high and sliced saw kerfs every 1-1/2" or whatever neatly divides your drawer width or depth (depending on which way the tray will slide). These strips are double-stick taped into position. You can see that these strips are just 2-by material and aren't even super accurate. Just be sure the top edge is sanded very smooth; I sealed mine with shellac and waxed them. I then cut some divider strips out of the 1/8" material from the back to an old stereo cabinet.
I then made a tray out of some hardwood stock with a simple captured bottom of 1/8" hardboard (the old back off a Wal-Mart drawer unit) and rabbeted corner joints. This tray should be wide enough in one dimension to fit well on the rails but not so snug as to have trouble sliding. The other dimension should be a little less than half the distance that it will slide; this makes sure you can access all of the lower area.
You can now get 33% more into the same space and still access it all easily.
You can also apply this technique without the dividers if you have odd shaped stuff or just don't feel like using them ;-)