I have a 27" iMac running Snow Leopard. In a wide range of applications I often need to use a straight edge to compare different values on charts, even sometimes charts generated by different applications. To do this I have made do with clumsy solutions like creating a floating and transparent Sticky or using Shift + Command + 4. I've also tried a small app called Crosshairs.
None of these things do what I really want. Ideally, I'd like a full screen crosshair centered on the cursor. Another way to think about this, it would give me a full height vertical and full width horizontal line that would show up over (on top of) any and all applications, just as a cursor does. Nice to haves it would give the coordinates of the display and have toolbar or right click access to turn it on or off.