I only recently discovered that metacity also supports composting. It doesn't offer the feature list of compiz, but, it does provide nice drop shadows and task switcher.
To use composting in metacity, set Visual Effect to None in the System/Preferences/Appearance dialog box. Apply the changes and your desktop will be running metacity.
Next, run this command to enable composting in metacity:
gconftool-2 -s /apps/metacity/general/compositing_manager -t bool true
and you can run this command to turn it off:
gconftool-2 -s /apps/metacity/general/compositing_manager -t bool false
Here's a simple snapshot showing the shadows:
If you really like tweaking your desktop, compiz works very well and provides lots of features. If you're happy with drop shadows and a nice tab switcher, metacity+compositing will do the job nicely.