New bookmarklet:
!grid (drag to Links-bar)

Click once to overlay the current page with a div containing a 50x50 grid pattern. Click again to hide. Should be handy for testing alignment, quick dimension estimates, etc. Tested in IE6 and Firebird 0.7.

(Suggestions for better grid patterns are welcome; grid9.gif looks OK here but looks a bit iffy on lighter backgrounds.)

Update: here's a new version using a different grid pattern, having alternating bright and dark pixels per P01's suggestion, and without the inner 10x10 lines:
!grid2