Peep!

Usage

Generate a Peep bookmarklet of your desired specifications below, then drag the Peep! link to your bookmarks bar.
Navigate onto a target site and click the Peep bookmarklet button. Peep will load and hamper your visibility on the loaded page. You can hit the P key to toggle the overlay on and off.

Requirements

Tested in Chrome 15 and Firefox 8.

Awesome! Let's do this!

Set it up!

Desired visible space

Bookmarklet (peepin' everywhere!)

Generated bookmarklet:

Peep!

Embed!

Copy the below HTML onto a page,

<script>var Peep=function(W){function Q(a,b,c,d,e,f,g,h){return X.globalCompositeOperation=a,X.fillStyle=b,X.setTransform(c,d,e,f,g,h),X}function I($){var D=W.document,b=D.body;b.appendChild(C=D.createElement("canvas")),S=$,Y=C.width=C.height=128,X=C.getContext("2d");with(C.style)left=top=0,width=height="100%",position="fixed",zIndex=900,pointerEvents="none";b.addEventListener("mousemove",function(a){V&&Math.abs((nx=a.clientX)-mx)+Math.abs((ny=a.clientY)-my)>3&&(mx=nx,my=ny,R(U))}),b.addEventListener("keyup",function(a){a.keyCode==80&&U(V=!V)})}function U(){C.width=Y;if(V){var w=innerWidth,h=innerHeight;Q("source-over","b",1,0,0,1,0,0).fillRect(0,0,Y,Y);with(Q("xor","w",h/w,0,0,1,mx/w*Y-H,my/h*Y-H))fill(arc(0,0,S*Y,0,360))}}var Z="equestAnimationFrame",R=W["r"+Z]||W["webkitR"+Z]||W["mozR"+Z],C,Y,X,S,mx,my,nx,ny,V=!(mx=my=nx=ny=0),H=.5;return I}(window);</script>

then use a link such as this to enable Peeping: