From Trilithon Software:
Trilithon GuideLines is an innovative screen guidelines application specifically for Mac OS X. This feature-rich product meets the needs of web developers and graphic designers for precision measurement and alignment.
First or Fourth Quadrant means that GuideLines measures the vertical axis either from the top down (fourth quadrant) or from the bottom up (first quadrant)
GuideLines starts up with an Initial Set of guides that trisect the screen in horizontal and vertical axes
Create New Guides by clicking the mouse in the 'track' at the edge of the screen. The new guide is created and you can move it to where you wish
Remove a Guide by dragging it out of the 'track'
Create Guide Grids by selecting Horizontal Grid or Vertical Grid from the Guides menu and then selecting the number of guides in the grid
Distribute Guides spaced evenly across the screen, or spaced evenly between the end guides
GuideLines Units include inches graduated in sixteenths, Picas, Points, Centimetres, Millimetres, Pixels, and Percentage of Length
Scale Factors include 100 percent (one for one), 200 percent, and 50 percent
Background Colour can be set to one of twelve different colours
Opacity Control can set GuideLines fully opaque, or can set them semi-transparent so that you can see through the guidelines to the underlying content
Move GuideLines individually or as a group, by dragging with the mouse
Real-Time Interactive Tracking of the mouse, plus high-precision display of current coordinates
Absolute or Relative Measurements. Absolute measurements displays the absolute location of the guide relative to the screen dimensions. Relative Measurements displays the location of each guide relative to the lowest guide for that axis. If you move a guide below the lowest one, the relative measurements change to match.
Show or Hide Coordinates Display. Coordinates display is on by default, but can be turned off if required. When guides are being moved, the coordinate display turns back on temporarily.
Group Guidelines so that they move as a group, rather than individually
Remember Settings across sessions
Remember Current Guides Configuration across sessions