Here is a tip that harks way, way back to 2006 — that’s the last time this little haxie from Unsanity was updated. ShadowKiller is available for free and, in spite of its age, nonetheless works just fine on my iMac running OS X 10.7.2.
ShadowKiller removes window and menu shadows in MacOS X. This changes the look of MacOS X completely, and speeds up its operation for slower Macs (such as G3).
Thereupon, XLR8 Your Mac has a discussion on the benefits of ShadowKiller on older (G3) Macs.
Rinse And Repeat
To remove shadows, just double-click ShadowKiller, your display will blink and shadows will be gone. To bring back shadows, double-click ShadowKiller again and, voila, they’re back.
To kill shadows on a more permanent basis, add ShadowKiller to Log In items in System Preferences.