Hoax Photo Test
I was 10 for 10 on level 1 but only 6 for 10 on level 2.  11:43:16 PM    

Mac OS X for Unix Geeks
Also on my reading list. Maybe not intellectual as the others listed below, but no less fun for me :)  10:55:36 PM    

Plumb Design Visual Thesaurus
This is the most beautiful java applet I've ever seen. Make sure you put it into auto-pilot mode (via the display options menu in the top of the window). Just incredible.  10:51:14 PM