A very short, very thin calibration object to measure build platform alignment
Successfully printing objects requires accurate build platform alignment. I find that 0.25 mm layers require leveling within about +/-0.05 mm across the entire platform.
This OpenSCAD program generates an object with bars exactly one thread tall and two threads wide. Skeinforge (or your favorite slicer) converts the STL file into G-Code that will produce a very short and thin plastic extrusion that reveals any misalignment.
Update: You must modify the OpenSCAD source to match your Skeinforge settings. The STL file is set for 0.25 mm thickness and 0.50 mm width, so it won't produce the correct G-Code for other printing parameters. It will (probably) print something that you can use for platform leveling, but the bars won't be exactly one-high by two-wide.
After printing, cooling, and removing the pattern, measure the thickness with calipers. Each bar should exactly match the Skeinforge layer thickness; if it doesn't, then you know which part of the platform to adjust.
My (extensively modified) Thing-O-Matic has a double aluminum plate build platform with three M3x0.5 mm adjusting screws at the left front, left rear, and right center mounting holes. Measuring the bar thicknesses near those screws directly indicates the required adjustment.
It also has a Z-minimum platform height sensor switch, so I simply adjust the screws to level the platform; my start.gcode routine measures and sets the correct Z=0 position. Without that sensor, you must adjust the screws to eliminate the tilt and set the height to match the layer thickness.
The overall printed width of each bar should match the Skeinforge W/T setting. For example, with 0.25 mm thickness and W/T=2.0, the bars should be 2 x 2 x 0.25 = 1.00 mm wide. This is very sensitive to the actual printed thickness.
Two tabs mark the +X and +Y axis directions so you can figure out where to make the corrections. The bottom surface will be shinier than the top, indicating which side was down.
More details and background on my blog at http://softsolder.com/2012/01/10/platform-level-test-pattern/.